The primary goal of the excitation control of synchronous generator is to realize the fast and accurate tracking of the output voltage to the reference value, and to keep the output voltage stable. The traditional PI control has some problems, such as insufficient adaptability to parameter change and poor anti-disturbance ability. In order to realize the rapidity and accuracy of the control, the discrete sliding mode controller is designed according to the state equation of the excitation control system of synchronous generator. In order to suppress the problem of error interference, a sliding mode error interference observer is designed. Through feedforward compensation and sliding mode control, the interference suppression and fast and accurate tracking of reference values are realized. Matlab simulation and semi-physical simulation verify that compared with traditional PI control, the proposed control method has stronger immunity and adaptability to parameter changes, and its output voltage tracking characteristics are better.