In a quasi-natural experiment of the expansion of the Yangtze River Delta, the panel data of 211 cities in China from 2008 to 2017 are analyzed. The difference-in-difference method is adopted to analyze the influence and the internal mechanism of the pollution emissions in related cities after the expansion of the Yangtze River Delta. The PSM-DID estimation, the parallel trend test, the synthetic control methods and the placebo test are adopted for the robustness test. It is shown that the expansion of the Yangtze River Delta would significantly reduce the pollution emissions in all related cities and the incumbent cities, but the reduction in the incumbent cities is greater than that in all related cities. The reduction of the emissions of the pollution is not significant in the new cities. In terms of the influencing mechanism, the pollution emissions in the whole Yangtze River Delta are significantly reduced through the scale effect, the technology effect and the structure effect, while the reduction of the pollution emissions is not so significant in the new cities due to the time delay. Therefore, it is necessary to strengthen the integrated regional development for the urban agglomeration and enhance the cooperation mechanism of the environmental pollution control in innovative regions, so as to realize the"win-win"goal-the regional economic development and the environmental protection improvement.

the enlargement in Yangtze River Delta  /  pollution reduction effect  /  internal mechanism  /  spatial differentiation