To address the risk posed by multiple flood sources in retention flood storage areas, taking the Baima section of the eastern retention area of Nan Si Lake as the study subject, employing a method mainly based on hydraulics and supplemented by hydrology, a flood evolution model was established for the retention area, considering the Nan Si Lake flood diversion, inter-regional rainfall runoff in the storage area, and slope runoff as the primary flood sources. Calibration of model parameters was conducted using actual data from the upstream catchment of Baima River and Malou, Simulations were run on the flood inundation progression in the Baima section under four combined scenarios considering the Nan Si Lake water levels at 36.79 m and 36.99 m, and precipitation frequencies of 50-year and 100-year return periods, The analysis focused on the impacts of different flood sources on the retention area from the perspectives of risk zone area, water volume, and inundation depth. The results indicate that when the flood diversion level of Nansi Lake is 36.79 m, the rainfall runoff between the flood diversion area and the flood storage area and the flood confluence on the slope have great influence on the flood storage area. When the flood diversion level of Nansi Lake is 36.79 m, the flood risk of Baima area is more affected by the water diversion of Nansi Lake, which is about twice of the rainfall runoff and the slope sink discharge.