Abstract: Surplus water risk of reservoirs is a key factor in dispatching operation of hydropower stations. In traditional reservoir scheduling, models were generally constructed to maximize power generation or income. Although an optimal solution to such models is theoretically possible, it always fails to account for the risk in determination of the reservoir water surplus and hence the objective functions so obtained will be practically useless. In this work, a water surplus estimate for a river basin in a given time interval was calculated first from the statistics of the basin’s historical runoff records, and then it was modified into the actual surplus of this interval by requiring it to satisfy a water balance equation. Finally, annual power generation of the cascade hydropower stations was maximized under a certain condition of surplus water risk. In a case study of the Wuxijiang River cascade hydropower stations, comparison with a traditional scheduling model shows that the new surplus risk model can produce scheduling schemes of better risk benefits for hydropower stations.