Under the background of building a modern socialist country in an all-round way and vigorously promoting the regional integration strategy, whether common prosperity can be achieved is the key to promoting high-quality economic development through a greater range of cross-provincial integration. However, there are few literatures pointing out the relationship between them. This article decomposes common prosperity into two dimensions: the growth rate that benefits the poor and the income gap between regions. Using big data from Baidu searches and social network analysis methods, a new measurement method for the level of integration is explored and applied to calculate the level of cross-provincial integration for 41 cities in the Yangtze River Delta from 2011 to 2020. Empirical evidence is provided to examine the impact of cross-provincial integration on common prosperity. The research results show that cross-provincial integration has a significant positive effect on common prosperity, and increasing the level of cross-provincial integration in the Yangtze River Delta can help increase the growth rate that benefits the poor and reduce the income gap between regions. And the promotion of common prosperity through cross-provincial integration is mainly achieved by pulling up rural and underdeveloped areas, while its effect on urban and developed areas is relatively weak. The “diffusion effect” rather than the “agglomeration effect” in cross-provincial integration is playing a dominant role. After using the replacement of independent variables, replacement of dependent variables, and instrumental variable tests, the conclusions are still robust. Based on the main conclusions, this paper puts forward targeted policy suggestions. The first is to expand the scope of cross-regional integration to achieve a wider range of common prosperity. The second is to guide the development of capital-saving technological progress and promote the improvement of labor productivity. The third is to encourage factors to migrate to the less developed in the region and boost the diffusion effect of regional integration.