Abstract: Based on the perspective of the integration of the Yangtze River Delta, this paper examines not only the evolutionary trend of the balanced development of the two major urban agglomerations in the Yangtze River Delta and the middle reaches of the Yangtze River, but also the promoting role of the polarization-spread effect of the Yangtze River Delta in affecting the economic growth of the middle reaches of the Yangtze River and regional balanced development, when using the Dagum’s Gini coefficient decomposition, the spatial externality theoretical model of economic growth, and the Spatial Autoregressive Model (SAR). The study draws four key conclusions: (1) The integration process of the Yangtze River Delta has not expanded the remaining gap in the development level of the middle reaches of the Yangtze River. In fact, the narrowing of the “gap between groups” of the two urban agglomerations is the main driving force for balanced regional development, while the effect of narrowing “internal differences” is relatively weak. (2) The spread effect of the Yangtze River Delta urban agglomeration on the middle Yangtze River urban agglomeration is greater than the polarization effect, which means that the positive spatial spillover effects in the Yangtze River Delta provides a significant boost to the economic development of the middle reaches of the Yangtze River and narrows the gap in development levels between the two major urban agglomerations in the Yangtze River Delta and the middle reaches of the Yangtze River, thereby promoting balanced development. Nevertheless, with the spatial correlation of the middle reaches of the Yangtze River being ignored, the diffusion effect of the Yangtze River Delta will be significantly overestimated. (3) The estimates imply that the higher the level of balanced development in the middle reaches of the Yangtze River, the more it will help the Yangtze River Delta’s spread effect to be fully realized. (4) Finally, we finds that the spread effect of different cities in the Yangtze River Delta is tied into the interaction of factors such as economic scale, industrial structure and geographical distance, presenting a nonlinear mode.