Abstract: Urban population growth has a very close relationship with urban pattern and industrial structure. Thus population control for megacities needs to consider these aspects. As evolution of urban pattern, megacities have turned from single center cities into metropolitan areas with multiple centers, as well as urban agglomerations. Urban rural labor market has broken through administrative boundaries. Meanwhile, megacities have finished industrialization and changed into a service occupied structure. Those changes make it necessary for rethinking population control for megacities from an angle of metropolitan area and urban agglomeration. Using the definition of Functional Urban Area by OECD, based on population census data and geographic data, we constructed a system for functional urban area for China. Based on this system, we define the area of urban agglomeration for megacities. As indicated by statistic data, population growth in first tier cities has turned from high rate into low rate even negative rate, with population scale reaching a steady state. Population issues in megacities embody mainly in structural imbalance. Thus the key for population control for megacities is to promote population extraction from urban center so as to facilitate spatial balance of population distribution.