Over the past few decades, internal migration in China has resulted in equity issues related to educational opportunities for migrant children. In an effort to provide equal schooling opportunities for migrant students, since 2001, the Chinese central government has implemented a series of policies to integrate migrant students into public schools. However, local governments were found to be implementing these policies in different ways, and the enrollment rates of migrant students in public schools varied from place to place. Existing research has largely investigated migrant children's educational outcomes, such as academic achievement. However, limited research to date has explored migrant students' well-being, which is considered an important outcome in recent decades by scholars from both within and outside China. This dissertation examines the urban-migrant gap in student well-being, specifically in mental health, sense of belonging, and school bullying victimization experience. Further, the study examines the role of school migrant composition in influencing the urban-migrant student well-being gap. This study utilized the China dataset of the Program for International Student Assessment (PISA) 2018, which contained 11,989 15-year-old participants from 361 middle schools, representing approximately 992,302 15-year-old students in Beijing, Shanghai, Jiangsu, and Zhejiang in China. The study utilized two-level hierarchical linear modeling (HLM) with appropriate student and school weights as the analytical method. Findings showed that migrant students had lower well-being than their urban peers in mental health, sense of belonging to schools, and school bullying victimization experience. Further, the school composition of migrant students is associated with the migrant-urban well-being gap. In particular, the school migrant composition significantly accounted for the between-school variation of migrantviii urban well-being gaps in student mental health; migrant students reported lower levels of mental health in schools with a higher proportion of migrant students. However, migrant students were found to have a higher sense of belonging when enrolling in schools with a higher proportion of migrant peers, relative to the migrant-urban gap in the sense of belonging for a typical school. The present study motivates the need for more attention on migrant students' school experience, alongside their academic outcomes. The different directions of how migrant composition relates to migrant students' mental health and school belongings raise the importance of considering school composition as an important factor in examining migrant education. As the number of migrant children arriving in urban centers continues to increase alongside China's economic development, policymakers and educational leaders should consider providing specific and targeted mental health services for migrant students; creating an inclusive public-school environment where migrant students feel connected and integrated; and enhancing necessary mental health resources for migrant students.