In order to improve the stability of distribution transformer output voltage, a new nonlinear automatic voltage regulation system of distribution transformer is designed. This paper introduces the design process of the step-up part of the nonlinear automatic voltage regulation system of the distribution transformer, which mainly includes step-up transformer, AC contactor and overcurrent protection device, and gives the circuit diagram. The design process of automatic control part is analyzed, including voltage detection circuit, A/D conversion circuit and control circuit. When the voltage distribution transformer is changed, through the acquisition of voltage microprocessor in the designated port output low level, using a small intermediate relay to control the opening and closing state of the contactor, to realize voltage control. According to the system performance requirements, the trigger circuit is designed, and the trigger circuit of the anti parallel thyristor module is given. The selection process of semiconductor device parameters of trigger circuit is analyzed. It is found that the voltage waveform is stable and the load waveform is almost no distortion after the thyristor is triggered. The design of the nonlinear automatic voltage regulation system is tested, and it is found that the design system can guarantee the stability of the distribution transformer output voltage.