Human adenoviruses (HAdVs) causes infections predominantly in infants and young children, and severe HAdVs pneumonia results in high lethality rate in children. CircRNAs are novel identified long non-coding RNAs and play important roles in gene regulation and pathogenesis of disease. To elucidate the roles of circRNAs in HAdVs pneumonia, we analyzed the circRNAs profiles of healthy children and children with HAdVs pneumonia including mild and severe cases, and identified 139 significant upregulated circRNAs in children with HAdVs pneumonia vs healthy controls, and 18 significant upregulated circRNAs in children with severe HAdVs pneumonia vs mild HAdVs pneumonia. In particular, hsa_circ_0002171 was found differently expressed in both groups, which may be taken as diagnostic biomarkers for HAdVs pneumonia and severe HAdVs pneumonia. To identify the underlying mechanisms of circRNAs in HAdVs pneumonia, we analyzed the transcriptome of children with HAdVs pneumonia and established circRNA-mRNA regulatory network. Enrichment of the differentially expressed (DE) target mRNAs demonstrated that DE genes between healthy controls vs HAdVs pneumonia were mainly involved in RNA splicing, while DE genes between children with mild and severe HAdVs pneumonia were mainly from the regulation of lymphocyte activation. Receiver operating characteristic (ROC) curve analysis suggested that hsa_circ_0002171 had significant value in HAdVs pneumonia and severe cases diagnosis. Taken together, profile of circRNA expression was altered in children with HAdVs pneumonia and severe cases. These results demonstrate that hsa_circ_0002171 as a potential diagnostic biomarker of HAdVs pneumonia and severe cases and may be a novel therapeutic target.