Power system simulation software PSS/E provides many interfaces to develop the application program. The dynamic process simulation system, with PSS/E power flow calculation as the core, is designed by Fortran API interface. First, the power flow calculation interface layer is achieved via API secondary developing and encapsulation to control the PSS/E power flow. Then a user-defined governor model is established, and frequency and governor simulation results interact with PSS/E power flow through the interface layer to achieve the dynamic process simulation. Dynamic predistribution technology of unbalance power during the simulation process is proposed, which makes the distribution of tie line power flow more reasonable during disturbance. Finally, the simulation results of IEEE 9-buses system and North China Power Grid verify that secondary development by PSS/E API interface can satisfy the requirements of real-time and high accuracy of large applications.