Rainfall usually leads to soil slope sliding and instability, which affects the safety of foundation pit, especially in the case of heavy rainfall. This study took the 7.20 Henan rainstorm as the background, where in the process of construction, after three days of rainstorm of 617.7 mm deep, the horizontal displacement of supporting structures of a foundation pit in Zhengzhou city increased by 6.3 mm. Therefore, it is of great significance to study the mechanism of deformation induced by rainstorm of foundation pits. Five numerical models considering different rainfall factors were developed to simulate the rainstorm process based on the monitoring data. The deformation mechanism and the effect factors of deformation on the foundation pit during rainstorm were analyzed, and some preventive measures were put forward for when the foundation pit engineering faces a heavy rainstorm. Under the action of the rainstorm, the supporting structure and the surface settlement had a signification deformation caused by the heavy rainfall on this typical foundation pit, and the maximum bending moment and maximum displacement of the supporting structure shifted up to different degrees. The main factors affecting the deep foundation pit of the metro by heavy rain are the steel strut falling off and the whole foundation pit filling with water, while the influence caused by the rise in the groundwater level, water standing load, and soil softening is small.