[Objective] The development characteristics of transient confined water were analyzed to provide scientific reference for the treatment and prevention of accumulation landslide.[Methods] Taking the Fanshantou landslide at Songyang County, Lishui City, Zhejiang Province as an example, the mechanism of transient confined water in the loose accumulation slope was simulated and studied. By analyzing the geological background conditions, hydrogeological conditions, the deformation and failure characteristics of the landslide area, and by comparing them with the rainfall data, the different rainfall conditions and the initial water lever of the slope were simulated using the GeoStudio numerical simulation software.[Results] When there were different layers with different permeability in landslide mass, heavy rainfall or high initial water level would promote the emergence and development of transient confined water and reduce the stability of slope.[Conclusion] The generation and development of external rainfall and transient confined water have an important impact on the formation of accumulation landslide. It could reduce the stability of slope and promote the appearance of landslide.