Abstract: In order to futher perfect the AC and DC speed-governing technology and improve the speed-governing performance of power drive system, in light of the coupled vibration of electric machine and drag machine in power drive system, the mechanism of nonlinear electromechanical coupled oscillation is analyzed as viewed respechvely from the mechanical nonlinearity in drive and the electrical nonlinearity in coverter control system. This kind of analysis and research is beneficial to making the reasonable strategy of vibration control.