Based on the first phase foundation pit engineering of a ship lock, the excavation deformation of foundation pit under the condition of precipitation was studied. Taking cutoff wall as the research object, MIDAS numerical simulation is used to study the influence of the cutoff wall on soil deformation around foundation pit at different depths. The results show that: under the condition of precipitation, the soil around the foundation pit is subjected to the upward frictional resistance of the cut-off wall, and the soil settlement decreases with the increase of the depth of the cut-off wall, and decreases with the increase of the distance from the cut-off wall. Small, the mainly affected area of the soil is 1 to 2 times the excavation depth; the soil at the bottom of the foundation pit produces elastic uplift deformation, which is mainly affected by the penetration of water flow and less affected by the depth of the seepage wall.