The complementary operation of wind, photovoltaic and hydropower systems has the potential to increase the integration of renewable energy sources into an existing grid. However, the high variability and forecast uncertainty of wind and photovoltaic power may pose potential risks to the grid and cascade reservoirs. In this study, the benefit and risk of the complementary operation of the wind-photovoltaic-hydropower hybrid system are evaluated. First, an optimal day-ahead complementary operation model is proposed to guide the daily operation of the hybrid system. Then, an optimal real-time load allocation model is developed to deal with power generation derivation resulting from the uncertainty of wind and photovoltaic power forecasting. Finally, the benefit and risk of the complementary operation of the wind-photovoltaic-hydropower system are evaluated. A case study is performed with the wind-photovoltaic-hydropower system in the Yalong River basin of China. The results show that (1) the total power generation, the power generation profit, and the utilization efficiency of electricity transmission lines are significantly increased, but the profit of hydropower stations is sacrificed to compensate for wind and photovoltaic power generation; and (2) the negative effect of the variability of wind and photovoltaic power generation on the power grid can be alleviated; (3) reliable power supply can be achieved, and power loss occurs mainly in the flood season and the maximum load loss is acceptable; and (4) it may lead to an increase in release variation, especially in the dry season, and thus flood control gates and hydropower generating units need to be operated more flexibly.