In a hybrid power system where renewable energy generation units and synchronous generators are jointly connected, the interaction between renewable energy generation units with different synchronization modes, and between renewable energy generation units and synchronous generators are all complex, deeply affecting the transient synchronization stability of the hybrid system. To reveal the dynamic coupling mechanism among different generation units under symmetrical grid fault and its impact on the transient synchronization stability of the system, this paper firstly establishes a mathematical model for transient stability analysis of a hybrid systems. Subsequently, the phase portrait method was used to analyze the impact of injection current of grid following renewable energy generation units, power instructions of grid forming renewable energy generation units on the transient synchronization stability of the hybrid system in detail.