The estimation of China's future food grain demand has become vital input for designing grain security measures. Addressing the population's age-gender and urban-rural structures under three fertility policies scenarios together with concerns for balanced diets, we established a multi-factor driven model to forecast China's food grain demand (including staple food grain and feed grain) during 2021-2050. The three scenarios are as follows; the two-child fertility policy for couples when either the husband or the wife is from a single-child family (scenario 1); universal two-child policy (scenario 2), and no limitations on the number of children (scenario 3). The results show that in scenario 3, China's food grain demand would peak in 2030 at about 329.3 million tons, about 3.7 million tons higher than that in scenario 2, and 104.7 million tons lower than that estimated with the traditional per capita method. These findings indicate that the demographic transition for fertility policy adjustment is not the main impacting factor of China's food grain security from 2021 to 2050. We might overestimate food grain demand by about 15 percent if we ignored each age-gender and urban-rural structure of the population. Then it may lead to an oversupply of grain and accumulation of stocks, which would generate about 1 billion RMB annual inventory cost burden. An important complement to the demographic strategy would come from the adoption of the proposed Dietary Guideline for Chinese Residents (2019). It can make people much healthier and save about 7.5 percent of China's food grain consumption, reducing the pressure scarce supplies of water and land in the country.