AimThis study aimed to test the relationship between length of residence and mental health in a school-based sample of migrant children who studied in migrant schools.MethodsA total of 7,296 rural-to-urban migrant children were recruited from 58 schools in Beijing and assessed by the State-Trait Anxiety Inventory and Children’s Depression Inventory.ResultsA quadratic relationship was found between mental health and length of residence. The results suggested that the scores for anxiety and depression were high during the initial resettlement after migrating and then decreased. However, after approximately 8 years, the scores increased. Our findings also showed a significant moderating effect of family socioeconomic status on the relation between mental health and length of residence.ConclusionThis study provided empirical evidence for a better understanding of psychosocial factors on the mental health of migrant children during the process of urbanization in China.