In order to analyze the stability of deep excavation slopes under heavy rainfall, the foundation pit of the Nanqiao diversion gate hub in Handan City was selected as the research background. Geostudio software was used to simulate two stages of excavation under working conditions of sudden extreme rainfall and prolonged heavy rainfall. Laboratory tests were conducted to obtain basic soil properties, and these data were used to investigate the instability mechanism and critical points of the slope under rainfall infiltration. The results show that soil moisture content is closely related to slope stability, and rainfall-induced moisture increase is a key factor leading to slope failure. It is also found that short-term intense rainfall and long-lasting rainfall produce similar effects; higher rainfall intensity causes faster failure, while longer duration leads to deeper influence and more significant lag effects. A triple-row soil nailing support system is designed based on the rainfall scenarios, which significantly improves slope stability. It is concluded that the proposed support scheme provides a useful reference for slope stability analysis and support design in deep excavation projects.