Abstract: In certain cases,residual soils appear to have high shear strength. But as they reach saturation,their shear strength reduces significantly to zero or very small. The measured data of the Shigu Mountain west passage project in Xiamen shows that the precipitation infiltration causes a significant increase in the deformation of the retaining piles of the foundation pit in the residual soil. This paper aims to explore the mechanism of precipitation intensification of the foundation pit deformation in unsaturated residual soil. It develops a numerical model in FE software PLAXIS and studies the responses of internal force of structure and stress stage of soil. The results show that rainfall infiltration can significantly increase the deformation of foundation pit in unsaturated residual soil and reduce the stability of the foundation pit. The reason is that rainfall infiltration increases the soil saturation,reduces the matrix suction. This leads to a reduction in shear strength. At the same time,rainfall infiltration leads to the increase of active earth pressure. Compared with the low permeability soil,the strength of the foundation pit bearing stratum in the residual soil with high permeability is more affected by rainfall infiltration. The stress path of the soil behind the wall shows that the shear force of the soil remains basically the same during the rainfall process,and the deformation of the foundation pit increases slightly. The dredging excavation after the rainfall makes the soil behind the pile reach the critical shear strength,which leads to the sharp increase of the deformation of the foundation pit.