China's domestic rural-to-urban migration has reached a stage where migration in family units is common, and basic data on the wellbeing of these families in urban settings are required for adequate social services. To obtain such data, we conducted a large-scale (N = 16,550) cross-sectional survey in Beijing of rural-to-urban migrant workers' children attending the fourth through ninth grades, including subsamples classified as poor (18.3 % of the total) and non-poor (16.1 %). Those in the non-poor group were comparable in affluence to the host population average. We found that overall physical health was good, with no difference between the poor and non-poor groups. Poor children, however, had worse mental health, and were at higher risk of mental health problems because of lower self-esteem, less family support, and lower monthly household income than non-poor children. This is the first study to analyze data obtained from poor children who constitute nearly one-fifth of the domestic migrant population in China. In addition to pointing out the higher level of risk for this group's mental health, our research suggests a need for countermeasures to improve self-esteem, maintain physical health, and emphasize family support. These results will help enable future hypothesis-verification mode survey research to illuminate general determinants and regulatory processes of health for migrant children, by providing large-scale survey data for a representative uppermiddle-income country for comparison with previously accumulated data from highincome countries.