Metropolitan Shanghai and Yangtze River Delta Area (YRDA) is undoubtedly the leading economic region in China, and it has become one of the most important population congregation areas. It is also widely considered as the sixth urban agglomeration in the world. Based on quantitative studies of the basic socioeconomic and demographic profile of population size, fertility transition, ageing, human resources and other factors, the author concludes that migration is the most important aspect of the population dynamics in YRDA. The paper studies two important forms of internal migration. First, it analyses the migration from outside YRDA into YRDA, or from inside YRDA to outside YRDA. Second, it analyses the migrations among different cities within YRDA. The paper explores how both forms of migration jointly influence the population distribution among cities, and determine changes of the urban structure. Furthermore, the paper makes a demographic projection of the future evolution of the urban agglomeration in YRDA based on estimates of urban specific fertility, mortality, and net migration. With projections of an urban agglomeration and five urban groups within YRDA, the author suggests that it is necessary to take a migration-led development strategy in YRDA within the context of the national population redistribution and regional development. It should strike a good balance between migration and regional sustainability, and it should push institutional reforms for adjusting and managing migrations in YRDA. Furthermore, the author suggests that differential development strategies should be advocated for different cities within YRDA, and it is necessary to strengthen regional coordination system for achieving an integrated regional development strategy in the area.