The economic development of city cluster of Changsha?Zhuzhou?Xiangtan is crucial for the development of Hunan Province as well as the central and western regions of China， and the effect of the implementation of the city cluster planning offers a kind of reference for other city clusters. In this regard， the paper uses the synthetic control method to assess the impact of implementing the planning on urbanization and environmental quality based on panel data of prefecture?level cities from 2000 to 2017. Analysis has showed that the implementation of city cluster planning has promoted urban?rural integration as well as the urbanization of Changsha?Zhuzhou?Xiangtan to some extent， but has weakened their environmental status， which is attributed to the unbalanced development among cities within the cluster.Thus， city cluster planning should pay more attention to formulating proper measures to promote balanced urban development， and should focus on urban?rural integration and ecological civilization. Morever， governments at all levels should enhance public governance and strengthen their supervision and governance responsibilities， so as to reach the targeted destination of planning.