Abstract: For a hydropower station with long-distance diversion tunnel, extremely large inertia of the tunnel flow leads to long fluctuation cycles, large amplitudes and slow decay of the water level in its surge tank. For those stations on an isolated grid, the regulation quality of transient process in small fluctuations using the traditional PID control law is poor and the fluctuations decay slowly. In this case, the cross-sectional area of surge tank often needs to be increased to improve stability, resulting in a higher project cost. If the stations connect to an infinite power grid, however, no previous studies on their stability and regulation quality are available. This paper constructs a detailed simulation model of hydraulic-mechanical-electric systems on infinite power grid, using the platform of Matlab software and its toolbox of Simulink and SimPowerSystems. This model is used to study the stability and regulation quality of hydropower stations with long diversion under small fluctuations. The results lay a basis for design and operation control of such hydropower stations.