An improved PID sliding mode control strategy with constant switching frequency for the voltage regulator of doubly salient electro-magnetic generator has been proposed in this paper. Next, operating principle of controller was described in detail and the stable condition of the system was deduced by Lyapunov function based on mathematical model and working principle of generator. Considering improved control law with constant switching frequency and stability analysis, the method about value selection of sliding mode coefficient was constructed. Finally, a field-circuit co-simulation model of the voltage regulation system for doubly salient electro-magnetic generator using PID sliding mode control was built by electro-magnetic finite element analysis technique and transient solver of control circuit. The steady state and dynamic response performance of generation system show that doubly salient electro-magnetic generator using PID sliding mode controller will acquire good steady and dynamic characteristics. Moreover, output voltage ripple and voltage drop of the generator is very small with increased load under PID sliding mode control.