Abstract: Expanding inertia from hybrid energy storage system (HESS) can improve the frequency stability of PV/HESS interconnected system effectively. However, the transient stability of system will be affected by complex inertia adjustment. In order to solve the transient stability problem caused by controllable inertia, the equivalent model of PV/HESS interconnected system was derived, and the effect of virtual inertia on the first swing of power angle was analyzed firstly. Then, the effect of adjustable virtual inertia on the damping characteristic was analyzed by establishing a small-signal model and using eigenvalue analysis method. Besides, a regional cooperative control of PV/HESS interconnected power system was proposed. Finally, a simulation model was established to verify the effectiveness of the proposed strategy. The results show that the strategy can take into account the frequency regulation, power angle stability and damping oscillations to restrain the sudden change in system frequency and to improve the frequency stability of system.