Abstract: Drought, floods, and extreme fluctuations in new energy output have severely impacted the stable and secure operation of hydro-wind-solar hybrid systems. A extreme operation scenario identification method and a medium-term complementary dispatching method under extreme scenarios are proposed in this paper. Standardized Power Output Index and Standardized Runoff Index are employed to identify extreme fluctuation scenarios in new energy output and extreme hydrological events, respectively. To address extreme operation scenarios, the medium-term complementary dispatching model is constructed with the objective of maximizing the minimum output. The model is transformed into a mixed-integer linear programming problem for solution. A hydro-wind-solar hybrid system in Southwest China is employed as a case study for method validation. The results suggest that the proposed method can effectively identify extreme fluctuation scenarios in new energy output and extreme hydrological events, and the medium-term complementary dispatching model can improve the power supply capacity and the daily output stability of the hydro-wind-solar hybrid system.