This paper investigates the multi-frequency dynamic performance of hydropower plant (HPP) under coupling effect of power grid and turbine regulating system with surge tank. Firstly, a model of grid connected HPP with surge tank is established and verified. Dynamic performance of grid-connected HPP with surge tank is evaluated by analyzing stability and dynamic response. Then, the number of frequencies of dynamic response is identified. The generation mechanism of multi-frequency dynamic response is revealed. Finally, the influence of system parameters on multi-frequency dynamic performance is studied. Guidance for actual applications of the obtained law and mechanism is proposed. The results indicate that the dynamic response of grid-connected HPP with surge tank has a feature of multi frequency. There are three frequencies for the dynamic response corresponding to three subwaves i.e. mass oscillation, water hammer oscillation and grid disturbance oscillation. Low-frequency mass oscillation is induced by water flow and level oscillation in subsystem of headrace tunnel and surge tank. Medium-frequency water hammer oscillation is induced by natural hydraulic oscillation in subsystem of penstock, turbine, governor and generator. High-frequency grid disturbance oscillation is induced by power grid. The coupling effect among penstock, turbine unit and power grid on water hammer oscillation is strong. (c) 2021 Elsevier Ltd. All rights reserved.