Abstract: In order to study the dynamic response of complex multistage gear transmission system under fault condition, the mechanical and electrical coupling dynamics model of complex multistage gear transmission system was established considering the flexible deformation of weak stiffness components such as gear ring, drive shaft and gear housing, as well as the nonlinear factors such as time-varying meshing stiffness, dynamic transmission error and electromechanical coupling effect during gear meshing. Taking the spalling fault as an example, fault modeling and implantation are realized by calculating the excitation force of the fault. Based on the simulation results, the dynamic response characteristics of the electromechanical coupling system under the condition of health and fault are compared. The results show that the stator current signal is modulated by the fault frequency of the gear in the fault state, and the fault characteristics in the frequency domain signal mainly exist in the form of modulated signal, and the envelope spectrum includes the fault frequency of the gear and its frequency doubling components. The fault characteristics in the electromagnetic torque frequency spectrum signal exist in the form of fault frequency at low frequency, frequency doubling and equal interval modulation side band at high frequency. Therefore, stator current and electromagnetic torque provide a new means to monitor the running state of gear transmission system.