Abstract: Storing as much floodwater as possible to meet the current and near future off-stream water demand on the premise of ensuring flood control and river ecological security, is the goal of the shallow lake floodwater utilization. For considering the uncertainties of inflows, potential water storage space and utilization patterns and determining the reasonable size of floodwater utilization, a method of combining simulation and optimization was used to establish an optimal model for moderate floodwater utilization sizes of shallow lake basins, and the model was applied in the Nansi Lake. The results shows that:① Utilization scales of shallow lakes in which utility patterns are mainly shores reverse regulation are limited by triggering water levels and capacities of pumping stations and off-stream water storage space. This limitation reflects obvious threshold effect instead of linear relation. ② The moderate size of lake basin-wide floodwater utilization is 363 million cubic meters per year and 156 million cubic meters in a flood season on a multi-year average while the pumping capacity is 48 meter per second and the trigging water level is 33.75 meters, the needed off-stream storage size is 480 million cubic meters. This study can provide a basis for the determination of the utilization size of Nansi Lake floodwater utilization and also a reference for the development of flood resources in other similar basins.