In order to solve the influence of partition excavation and internal support conversion on surrounding environment in open excavation of subway tunnel. Based on the second phase of open-cut section of Xisha Section of Zhengzhou Metro Line 5, this paper studies the deformation law of foundation pit enclosure structure and surrounding environment in the process of pit partition excavation and internal support system transformation. It is preliminarily obtained that adjusting the width of each small foundation pit, the excavation and replacement sequence have a very significant impact on the retaining structure and the surrounding environment. The effect of changing the depth of common retaining wall on retaining structure and surrounding environment is relatively weak. In practical engineering, when the surrounding protection requirements of one side of small foundation pit are higher, it is recommended to set the middle partition wall on the side. If the position of the middle partition wall and the excavation depth of the small foundation pit can not be adjusted, and the protection requirements of the large width of the small foundation pit are higher, it is recommended to excavate the small foundation pit with small width and shallow depth first, and then excavate the small foundation pit with large width and deep depth, so as to reduce the influence on the retaining structure and the surrounding environment.