The vibration and noise characteristics of electric vehicles are quite different from those of internal combustion engines. The calculations of the internal excitations of the motor-reducer integrated drive system are the foundation of dynamic analysis. In this paper, the internal dynamic excitations of a certain electric powertrain in rated revolution are calculated by a theoretical analysis and numerical simulation method on the basis of gear dynamics and electromagnetism, including the electromagnetic radial force, torque ripple and external circuit in the motor, and the gear time varying meshing stiffness, meshing error and meshing impact in the gear system. This research lays the foundation for dynamic analysis of the electric powertrain.