Abstract: Rainfall is usually easy to cause sliding and instability of soil slopes. At the same time, it also has a great influence on foundation pits, especially for those in soft soils. In the absence of other construction, after three days of rainfall of 247 mm deep, the horizontal displacement of supporting structures of a foundation pit in Tianjin increases by 13.75 mm. Therefore, it is of great significance to study the mechanism of deformation induced by rainfall of foundation pits in soft soils. Firstly, the experiments on the influences of rainfall on relevant model are conducted by using Plaxis2D. The influence mechanism of rainfall on the deformation of supporting structures of the foundation pit in soft soils is analyzed. The influences of intensity, duration and amount of rainfall on the deformation of supporting structures are analyzed, and the sensitivities of the excavation depth, the initial displacement of pile top, the form of supporting structures to rainfall are studied. The main factors affecting the deformation of supporting structures are the increase of the residual soils outside the pit, the softening of the soils in the pit and the fluid-solid coupling. The amount of rainfall is a factor to have a large influence on the deformation of the supporting structures of the foundation pit. The displacement increment of the supporting structures caused by rainfall is most affected by their initial displacement, while the depth of foundation excavation has small influences on their deformation increment under the same rainfall conditions.