Wound-field Doubly Salient Generator (WFDSG) has the outstanding advantages of simple structure, high reliability and controllable excitation. However, there exists obvious torque ripple in generator’s input torque, which has restricted its applications. Based on the concept of electromagnetic torque of WFDSG, the nonlinear formula under no load condition has been derived. Furthermore, it is proved to be reasonable theoretically in comparison with linear and approximate nonlinear formulas. Based on the 12/8 salient prototype, the correctness of nonlinear formula has been verified by comparing the results of virtual displacement method with the results of finite element analysis. At last, the simulation results have been validated by experiments.