Excitation control system is an important part of the synchronous generator. Its main function is to provide a controllable DC current (voltage) to the stator winding of the synchronous generator to maintain the constant voltage of the terminal, which meets the needs of the synchronous generator. Through the genset control system model, the unit simulation model could be calibrated in time, and the abnormal response could be checked for early warning, so as to effectively improve the stability of the system operation. A simulation model of singlemachine infinity system with external simulator as excitation regulator was built in MATLAB. The model simulated the output of the excitation system when the PID main loop and the power system stabilizer (PSS) auxiliary loop were anomalous. The output was compared with the theoretical value. Through comparison, it is confirmed that the proposed model can determine the cause and location of the fault in time and thus help to repair the faults.