The large number of photovoltaic power generation will change the damping characteristics of power system and make the oscillation easier to occur. In order to study this problem more simply and effectively, considering the main circuit and control strategy, a small signal model of photovoltaic system is established. The system model is reduced by ignoring the fast dynamic factors, and four reduced models are obtained. The oscillation modes and their sensitivity to state variables are obtained by eigenvalue analysis. It is found that the lower order of model, the simpler calculation, but the fewer the oscillation modes can be retained. When analyzing the influence of factors on oscillation modes, the appropriate reduced model is selected according to the participation factor and the reservation of oscillation modes in reduced models, and the correctness of this method is verified in MATLAB. The simulation results show that the proposed method can not only correctly reflect the oscillation modes of photovoltaic power generation system and the influence of each influence factor on the oscillation modes, but also have significant advantages in simplifying the analysis process and improving the calculation efficiency.