A mathematic model based on a doubly fed generator and its rotor excitation control system, which is related by the phase angle between the stator and rotor voltage, is established in this paper. Meanwhile, a unified vector control model in synchronous rotating reference frame is deduced. The model not only reflects the relationship between the machine parameters, hardware structure and parameters of the rotor excitation control system but also exhibits that the doubly fed generator has the common feature of induction machine and synchronous machine as well as the technical superiority of the doubly fed generator in the variable speed constant frequency (VSCF) generation system. The simulation results indicate that the proposed model is suitable for the study on doubly fed excitation control strategy and design of the excitation control system