In the context of “carbon peaking and carbon neutrality”, the large-scale integration and consumption of wind and solar resources are an inevitable trend in future energy development. However, as the capacity of wind and solar power integration increases, the power system also requires more flexible resources to ensure secure operations. To investigate the flexible regulation role of hydropower in the system, this study focuses on the downstream stations of the hydro-wind-solar-pumped storage clean energy base in the Yalong River basin as the research subject. Considering flexible regulation capabilities, the study conducts day-ahead optimized operational strategy research for the complementary system. Firstly, to address the challenges of site selection and high costs associated with independent pumped storage, steady-state models considering hybrid pumped storage stations for cascade hydro-wind-solar-pumped storage are established. To address the limitations of traditional models with low predictive accuracy and the subjective selection of LSTM hyperparameters, PSO is used to optimize the parameters of LSTM and the best parameters of LSTM is used to forecast the output of wind and solar power. Subsequently, in order to fully harness the flexible regulation potential of complementary system, a multi-objective optimal dispatching model is constructed to consider the economic benefits and flexible regulation margin of the complementary system in the day-ahead time, and NBI method is employed to solve the constructed multi-objective problem, which can obtain the pareto optimal solutions with even distribution. Finally, case studies are conducted based on the actual conditions of the Yalong River basin. Through analyses of different scenarios, the effectiveness of the proposed model and the supportive role of pumped storage in enhancing system flexibility are validated. The results demonstrate that the proposed approach not only balances system profits but also fully exploits the flexible regulation potential of the system, ensuring the stable operation of the system.