To meet more requirements of a photovoltaic generation system with the increase of installed capacity, virtual inertia control and low voltage ride-through are added to enable the system to have frequency and voltage regulation. Given a photovoltaic generation system with virtual inertia control and low voltage ride-through, an extended nonlinear model of phase locked loop (PLL) is established for transient analysis. Based on the model, an equal area criterion is used to analyze the transient synchronization process of the system before a fault, when a fault occurs and after fault removal, so as to determine the factors affecting system stability. Then the equal area criterion is used to analyze the influence of voltage sag depth, virtual inertia controller parameters, phase locked loop proportional coefficient and integral coefficient on the transient stability of the photovoltaic system. Simulation results show that the proposed method can effectively analyze the influence of various parameters on the synchronization process of a photovoltaic power generation system, and verify the correctness of the theoretical analysis. This work is supported by the Key Project of National Natural Science Foundation of China (No. 51637005).