This thesis aims to further our understanding of migration, family separation and wellbeing, by conducting a systematic examination of how family separation impacts the wellbeing of the left-behind children and spouses in the context of the great internal migration in China. Drawing on the life course perspective, this thesis investigates the internal migration phenomena as the sociological, phycological and demographical issues, and studying the wellbeing of the affected individuals across the life course in the collective contexts such as families, couples, cohorts as well as its cultural frames and institutional conditions, relating the analysis in the macro, meso, and micro levels. Employing nationally representative datasets, this thesis conducts three empirical studies to examine the links between the absence of parent/spouse and wellbeing at different stages of the life course – childhood, young adulthood, and the entire span of marital adulthood, by taking a long-term view and considering the timing of the life events. Firstly, the thesis finds that parental migration and absence negatively affect the psychological wellbeing of children, and the early parental absence has a lasting negative impact on children’s psychological wellbeing in their youth. Secondly, the thesis finds that the experience of parental absence in childhood has a lasting impact onto adulthood, and documents that childhood parental absence has a negative influence on the mental and physical health but positive influence on the cognitive ability of individuals in their adulthood. Thirdly, this thesis documents that spousal migration and absence negatively impacts the physical health and mental wellbeing of the left-behind spouses, with a larger impact on the left-behind women’s physical health and the left-behind men’s mental health. Based on the findings, the thesis also explores some mechanisms that moderate the relationships. This thesis enriches the literature in migration, family separation, and wellbeing, and provides implications for policy practices and future research.