Aiming at the problem that the operation state of three-phase PWM rectifier in AC microgrid is easily affected by load disturbance and filter capacitor parameter perturbation, a sliding mode control method of three-phase PWM rectifier based on nonlinear disturbance observer is proposed in this paper. Firstly, the load disturbance and filter capacitor parameter perturbation are regarded as lumped disturbances, and the nonlinear model of voltage loop is constructed. Secondly, a nonlinear disturbance observer is designed to realize the online estimation of lumped disturbances. In order to achieve active anti-interference control and sliding mode chattering suppression, the estimated lumped disturbance is introduced into the sliding mode surface, and a new sliding mode surface with disturbance estimation is obtained. Finally, the voltage outer loop sliding mode controller is designed based on the sliding mode control theory, and the stability is proved by Lyapunov stability theory. The dynamic performance test results show that the nonlinear disturbance observer can estimate the lumped disturbance quickly and accurately. Compared with the double-loop PI control, the transition process time of the proposed control is reduced by 60%, and the voltage drop is reduced by 54%. When the filter capacitor parameters change, the dynamic performance of the proposed control remains good. The steady-state performance test results show that compared with the traditional sliding mode control and the double-loop PI control, the total harmonic distortion of the current of the proposed control is reduced by 70% and 44% respectively, and the sliding mode chattering is effectively suppressed. The test results show that the proposed control method has strong anti-load disturbance ability and good robustness of filter capacitor parameters.