Over the past three decades, social change has led to unprecedented massive internal migration in China. An increasing number of people leave their original places of residence to work and live in other cities or provinces to improve their lives, such people are called internal migrants (IMs). The number of IMs exceeded 240 million in 2017 . IMs are vulnerable to social exclusion in their destinations , which prevents them from accessing local National Essential Public Health Services (NEPHS) as equitably as natives . Social exclusion has become a risk affecting the health of IMs .
However, the situation has been improving in the past decade. China initiated the NEPHS project in 2009, the project is provided free of charge by the government to all residents, including IMs who have lived there for more than six months . NEPHS includes health records, health education, immunization for children, and chronic disease management et al. Since then, the National Health Commission (NHC) has successively introduced measures to strengthen the equalization of NEPHS[6–8]. Efforts have paid off, and the IMs' NEPHS utilization level is rapidly improving . The new situation promotes the government's working mode to change from flooding to precise , and the focus has gradually shifted to how to achieve the NEPHS equalization within the IMs. Recent studies have confirmed that sex, education, community type, migration range (MR) and regional economic development level (REDL) have significant impacts on IMs' NEPHS[3, 9, 10–15]. Studies have consistently concluded that the NEPHS utilization of inter-provincial IMs is lower than that of intra-provincial IMs [9, 10, 13–15]. However, opinions on the relationship between REDL and IMs' NEPHS utilization are inconsistent. Some believe that the IMs' NEPHS utilization in high income provinces (HIPs) is lower than that in low-middle income provinces (LMIPs) [9, 12–15], but others do not think so .
Existing studies use indirect indicators to measure REDL, and the scales are different. It is urgent to adopt a new method to clarify this inconsistency. In addition, previous studies have viewed MR and REDL as independent factors, without considering their possible interaction, nor have they made a detailed discussion on the mechanism behind the two factors. The household registration status still affects and determines the urban class structure and social integration, and the social and economic conditions between the registered population and migrants as well as within the migrants are heterogeneous [16, 17]. The primary relationship such as geography is still a basis for the IMs' social network reconstruction . However, the hometown consciousness is exclusive, people within hometown are easily accepted, while people outside it are excluded. This hometown can be a village, a city or a province . So inter-provincial and intra-provincial IMs may have different socioeconomic status (SES). On the other hand, HIPs attract a large number of IMs. In order to control the population, the HIPs adopted a stricter household registration access system. Compared with LMIPs, the IMs face stronger institutional exclusion in HIPs. It can be inferred that MR and REDL may have significant impact on IMs' social capital (SC).
SC is a social determinant of health , and it can also have an impact on health services accessibility . Different types of SC affect health services utilization by influencing the availability of health services in communities, the availability and effectiveness of outreach resources between health-care providers and communities they serve, and care-seeking behavior of individuals in those communities . Migration means a loss of the original social network and a reduction of social participation in the new environment . However, the SC that migrants have in the destination is more important for their access to local health services [24, 25]. Guo et al.  had pointed out that lack of information is the biggest obstacle to IMs accessing NEPHS. SC can influence health information through three mechanisms: increased information exposure, enhanced seeking abilities, and reinforced health culture or norms embedded in social networks . Therefore, SC may play an important role in IMs' NEPHS utilization, few studies have confirmed this [28, 29]. Carpiano and Moore  suggested that SC and health knowledge base can be better served by asking three foundational questions of (a) how, (b) for whom, and (c) in which contexts does SC work, rather than focusing primarily on whether SC provides some universal health benefit. However, previous studies on questions b and c are far from complete.
Surveys show that the SC of IMs is significantly lower than that of local residents [31, 32], but few studies have explored the distribution of SC among subgroups of IMs. The relationship between SC and NEPHS utilization of IMs in different subgroups remains unclear. Two models have been put forward by Cohen and Wills  to explain the mechanisms by which SC influence health outcomes: the stress-buffering model posits that social ties are related to well-being only for persons under stress, whereas the main effects model proposes that social ties have a beneficial effect regardless of whether individuals are under stress. Fried and Tiegs  have proved that both effects can exist at the same time. Uphoff et al  proposed three paths by which SES could affect the relationship between SC and health: (1) A more significant SC benefit on the health of disadvantaged persons in society, and no effects or limited health benefits for those in positions higher up in the social ladder. (2) People with a low SES will generally have less SC, and the SC available to them cannot be used effectively for health benefits. (3) SC might benefit the better-off in society while excluding people with a lower SES. Which of these conclusions apply to China's IMs remains to be verified.
Given MR and REDL may have a significant impact on the IMs' SC, and SC is closely related to NEPHS utilization, we speculate that MR and REDL affect the IMs' NEPHS utilization by affecting their SC. To test this path, a sample of the China Migrant Dynamic Survey (CMDS) in 2017 was adopted, and we want to verify two hypotheses: (1) Both the SC and NEPHS utilization level of IMs are significantly affected by MR and REDL; (2) The relationship between SC and NEPHS utilization of IMs is significantly moderated by MR and REDL. This study is the first to explore the impact of MR and REDL on IMs' access to public health services from the perspective of SC based on a national sample. Our study can provide evidence for the Chinese government to deepen the equalization of NEPHS for IMs, and can also provide references for other regions to deal with the health problems of IMs.