Aiming at the problems of complicated control structure and slow dynamic response speed of three-phase PWM rectifier direct power control under the double-loop control structure, and considering the applicability of control algorithms applied in microprocessors, a single-loop discrete sliding-mode direct power control method based on the discrete nonlinear disturbance observer for three-phase PWM rectifier is proposed. Firstly, the rectifier DC-side power model and the grid-side active power model are considered as a whole to obtain the DC-side-grid-side active power model. Second, a discrete nonlinear disturbance observer is designed and a novel sliding mode surface containing power perturbation estimation is constructed to improve the power perturbation resistance of single-loop discrete sliding mode control and reduce its power tracking error. Finally, the dual-loop PI control, the traditional discrete sliding mode control and the control proposed in this paper are compared on a processor-in-the-loop testbed. Compared with the dual-loop PI control, the regulation time and voltage dips of the proposed control are reduced by 90%, 35.7% and 91%, 41% for resistive load surge and constant power load surge, respectively. During steady state operation, the total harmonic distortion rate of the current of the proposed control is reduced by 36% and 15% compared with that of the traditional discrete sliding mode control and dual-loop PI control, respectively. The test results verify the effectiveness and superiority of the proposed control method.