Abstract: With the large-scale integration of new energy into the power grid, the unstable primary frequency regulation performance of hydropower units and their vulnerability to grid assessment have become prominent issues. Energy storage systems can provide effective auxiliary support for such units. Existing studies mostly focus on joint simulation and control strategies, while lacking in-depth analysis on the cooperative mechanism and stability of hydropower-storage systems. To address this gap, this paper establishes a simulation model for hydropower-storage primary frequency regulation, analyzes the system stability mechanism, clarifies the adaptation relationship between hydropower units and energy storage, and proposes a cooperative control strategy. Simulation results verify that energy storage can effectively improve the frequency regulation stability of hydropower units. The proposed strategy can meet grid assessment requirements and provide theoretical support for the engineering design and operation optimization of hydropower-storage cooperative systems.