Since the reform and opening-up policy was implemented in 1978, China has loosed its household registration policy and rapidly advanced the construction of urbanization. The pattern of population migration has shifted from planned migration to autonomous migration. The Jiangsu-Zhejiang-Shanghai region has experienced a significant increase in the scale of the in-migration population, thence it is highly meaningful to research its driving mechanisms. Based on the in-migration data in the sixth population census, a multivariate linear regression model was constructed by the independent variables, such as the resident population, per capita GDP, per capita employment wage, per capita medical beds, population with a college degree or above and distance. Furthermore, the hierarchical Bayesian theory was introduced to reduce the indeterminacy of parameter estimation. The results show that economic factors serve as the primary driving force. Population factors, the education factor and distance factor also play important roles, while the medical factor has a relatively weak effect.