Seasonal pumped storage (SPS) is a sustainable and effective energy storage solution that can mitigate the seasonal fluctuations of renewable energy sources and provide flexibility to power systems. Despite its huge potentials, the operational mechanism of SPS, particularly for the multi-energy complementary operation, remains poorly understood. In this paper, we propose an optimal scheduling model of a regional power system with SPS and evaluate its potential contribution to energy efficiency, power system stability, and environmental impact. A case study in Qinghai, China, demonstrated that SPS effectively managed seasonal fluctuations of renewable energy sources, improved hydropower generation during the dry season, and stored excess power from renewable energy sources and the main power grid. Furthermore, our analysis revealed that SPS significantly improved energy efficiency compared to traditional pumped storage by increasing system energy efficiency by 11.21%, reducing wind and solar power by 72.97%, and lowering carbon emissions by 94.41%. Overall, SPS has immense potential in providing power system flexibility and enhancing the decarbonization of power systems.