Based on the panel data of Chinese cities from 2006 to 2016 and taking the enlargement of Urban Cluster of Yangtze River Delta as an example, this paper empirically studies the impact of enlargement of urban cluster on green economic efficiency was studied by using the differences-in-differences method. The results show that enlargement of the Yangtze River Delta urban cluster has a significant effect on the green economic efficiency of the original cities in the urban cluster, but has no significant effect on the green economic efficiency of the new cities. The robustness test shows that the conclusion is reliable. Mechanism test shows that enlargement of urban cluster does not affect the technical efficiency of the original cities, but promotes the technological progress of the original cities by improving the level of market integration, and then improves the green economic efficiency of the original cities. Enlargement of urban cluster improve the technical efficiency of new cities by reducing the level of functional specialization. Enlargement of urban cluster promote the technological progress and reduce the technical efficiency of new cities by improving the level of market integration. Because the effect of technological progress and technological efficiency reduction cancel each other, enlargement of urban cluster does not affect the green economic efficiency of new cities in urban cluster. In addition, the enlargement has a greater effect on the green economy efficiency of the original cities with large scale, high innovation capacity and high economic development level.