Abstract: With the proposed goal of "dual carbon" and the gradual construction of a new power system, complementary management of wind, solar, and hydropower has become an inevitable trend. However, methods for calculating the guaranteed power output of hydro-wind-solar complementary systems are still in scarcity. In this paper, taking the clean energy base in the upper Yellow River Basin as a case study, four methods for calculating the guaranteed power output of hydro-wind-solar complementary systems—namely the superposition method, trial calculation method, empirical frequency method, and theoretical frequency method—are proposed, and the results are verified based on a deterministic optimal operation model. The results show that the guaranteed power of the hydro-wind-solar complementary system based on the trial calculation method can effectively reduce the fluctuation of system output while maintain a high level of energy production, performing best among all the schemes. This study analyzes the advantages and disadvantages of the four methods and provides a theoretical basis for the scientific planning and scheduling of hydro-wind-solar complementary systems.