Multi-phase self-excited synchronous motor has the advantages of large starting torque, no need for permanent magnet, and adjustable air gap magnetic field, which is widely applied to the large torque demand under short-term special conditions such as low speed climbing. As a new type of motor, its excitation mechanism mathematical model is not clear, which directly restricts the development of the motor control system. Therefore, the excitation of the motor under steady state is studied. The basic structure and high-frequency excitation principle of the self-excited synchronous motor are analyzed. The mathematical relationship between the high-frequency current injected into the stator winding and the excitation current generated on the rotor excitation winding under steady-state conditions is studied. On this basis, the electromagnetic torque expression is obtained. The motion equations of the motor in stationary and rotating coordinate systems are established. Finally, the MATLAB/Simulink simulation model is built under the rotating dq coordinate system and the Maxwell finite element simulation results are compared and analyzed. The results verify the correctness of the mathematical model.