Taking the deep foundation pit with a suspended impervious curtain as an example, ABAQUS was used to establish the three-dimensional fluid-solid coupling model considering the whole process of grading precipitation and excavation in order to analyze the influence and unfavorable factors of precipitation on the deformation of the foundation pit. Then the depth of first-stage precipitation and the impervious curtain were analyzed. Results show that seepage and excavation support have coupling effects. The lateral displacement increment of support piles caused by precipitation varies with the conditions of excavation support. The ground settlement caused by precipitation is influenced by soil consolidation and the lateral displacement of support piles caused by precipitation. The scope of ground settlement is larger than that of empirical prediction, and precipitation-induced settlement accounts for 48% at the maximum point of settlement. The first-stage precipitation is the most unfavorable to the foundation pit of all stages of precipitation. The initial lateral displacement of support piles increases rapidly with the increase of the first-stage precipitation depth, so that the growth of the final maximum lateral displacement of support piles and ground settlement is exponential. The impervious curtain can effectively reduce the water level outside the foundation pit drop and control ground settlement. The maximum ground settlement and the scope of ground settlement decrease as the depth of the impervious curtain increases. There is an optimal curtain depth which makes ground settlement stabilize after the curtain depth exceeds the optimal depth.