Based on the fifth census, we use the core-periphery model to inspect the migration among cities in the Yangtze Delta megalopolis. It is known that dynamics of agglomeration and diffusion in the megalopolis manipulate developments of cities. The strength of agglomeration is a major force in the megalopolis, and the pursuit for economic benefits is a basic reason for population clustering. Meanwhile, the inter-city dispersion is mainly a non-class diffusion. Also, dynamics of population in the periphery differ between north and south. The north in the Yangtze Delta has a denser interaction with the core than the south. But when applying the gravitation model to analyze the spatial interaction, we find that the decay of distance friction between south and core is smaller than the north, which indicates that the relation between south and core is increasing. In intra-cities there is a dense population flow, and the population urbanization and the migration between blocs are two main trends. Some policies should be adjusted to fit an integral development in the megalopolis.