The Photovoltaic with characteristics of low inertia and weak damping is connected to the grid on a large scale, which brings about adverse effects on the grid stability. In this paper, the inertia and damping mechanism of photovoltaic system under typical control strategy is studied, based on the modelling idea of multi-time scale and electric torque method, the source and mechanism of inertia and damping of the system are clarified from the physical mechanism level. The results show that the inertia and damping effect of photovoltaic system can be derived from many links including energy storage elements. Meanwhile, the structural, steady-state operating point and control parameters can influence the inertia, damping and synchronous equivalent parameters of the system. Adjusting control parameters is the easiest way to change the inertia and damping characteristics of the system. The correctness of the above theory and influence law are verified by the MATLAB/Simulink.