In this article, the equivalent power angle transient response of the grid forming (GFM) during fault are derived in detail. Based on the control system of GFM, the power angle characteristic equation is proposed, which investigates the effects of the GFM system’s control parameters and the operation status on the general output characteristics of the system. Then the transient synchronization characteristics of the GFM system are derived from the equation. The relation between the actual value and the instruction value of the output power is used as the criterion to judge transient stability of the GFM system. Considering the detection delay, the transient synchronization mechanism of the GFL grid-connected system is studied by analyzing operating trajectory of equivalent power angle (EPA) and using the equal area criterion (EAC). In addition, the sequential switching control schemes from normal stage to fault stage are introduced. Moreover, the stability assessment method based on the EAC is proposed, and the transient synchronization processes of different time sequence stages are studied. Finally, the theoretical analysis is verified by the time domain simulations.