Population movement is the main carrier of inter-city factor flow and resource alloca-tion. It is also one of the main forms of regional network construction. As the Yangtze River Delta integration has become a national strategy, in order to promote inter-city population movement and regional integration, it is of great importance to recognize the pattern of population movement and analyze the influencing factors. This study focuses on the inter-city daily mobility within 48 hours, one of the important components of population movement, based on the Weibo sign-in data. It analyzes the pattern of inter-city population movements in the Yangtze River Delta, applying the gravity model to test influencing factors, from perspectives of movement cost and city characteristics. The results indicate that: (1) population movements in the study area have multiple cores, and connections between cores and their hinterlands are relatively balanced. In addition, there are three communities in this region, and Shanghai, Suzhou and Hangzhou belong to the same community. (2) Strong population movements occur within each province, and population movement systems of the three provinces are different. Zhejiang and Jiangsu provinces have formed a relatively mature multi-level population mobility system, while the population mobility system in Anhui Province needs to be improved. (3) Movement cost and city characteristics complement the inter-city mobility model shaped by physical distances, and compared with movement cost, city characteristics have a greater impact on population movement. (4) Compared with the inter-province movement, population movements within each province are stronger, and are more likely to occur between cities with different cultures. The greater the differences between the two cities, regarding economic scale and administrative level, the stronger the population movements between them. Besides, differences in industrial structure will inhibit intercity mobility, while differences in education level can promote mobility. This paper expands the applications of the gravity model, analyzes inter-city daily movement mechanism, and provides references for understanding the process of the Yangtze River Delta integration, as well as optimizes policies from the perspective of daily population movement.