In order to solve the model complexity and analysis difficulty of electric system joined up by photovoltaic power system, a simple and practical model reduction method of photovoltaic power system is proposed. Considering the main circuit and control strategy, a small signal model of photovoltaic power generation system is established. On this basis, the multi-time scale characteristics of the system model are decomposed to determine the fast and slow states. The correctness of the fast and slow states decomposition is verified by the relative velocity estimation method, which avoids the inconsistency of the stability of the system before and after the reduction caused by the improper selection of perturbation parameters. By ignoring the fast dynamic factors, the order of the model is reduced, and the zero-order approximation model and the first-order approximation model are obtained by adding correction terms. Finally, the dynamic responses and calculation errors of the reduced order model, the corrected model and the original model are compared in MATLAB. The results show that the reduced model and the correction model can correctly reflect the dynamic response of the detailed model under small perturbations, and have obvious advantages in improving the calculation speed and simplifying the analysis process.