Abstract: In this paper, excavation of the Pudong wiring segment was studied belonging to the South Xizang Road Tunnel crossing the river in Shanghai. The plane finite element model of laddershaped pit was established. In the case of middle wall’s different insertion ratio, the lateral displacement curves of three walls and mechanical behavior of the pit’s lateral deformation were analyzed. The top of the middle wall tends to the pit’s outside, and the middle and bottom of the wall tend to the pit’s inside. Increasing insertion ratio of the middle wall and lateral displacement of the middle wall gradually decreased. Because the insertion ratio of the middle wall was increased, the whole lateral displacement of left wall tends to decrease, and the location of maximum lateral displacement was closed to the depth of the pit’s excavation face. When the middle wall’s insertion ratio increased, the support state of the right wall tends to improve, but not obviously. When ensuring the pit’s security and economy, a more reasonable insertion ratio of the middle wall is 2:1 to 3:1.