Abstract: In view of problems of complex structure and control, high cost and big volume of contactor and charging resistance existed in traditional pre-charge circuit of single-quadrant frequency converter, a new type of pre-charge circuit of single-quadrant frequency converter was designed. The circuit uses three-phase half control rectifier bridge to replace diode rectifier bridge of traditional pre-charge circuit of single-quadrant frequency converter and takes off contactor and charging resistance. When the frequency converter starts, trigger angle of the three-phase half control rectifier bridge decreases rhythmically from 120° to 0°through synchronization circuit and single-chip microcomputer control circuit, so as to make charging current of capacitance in allowable range. The test result showed that the circuit has simple structure, reliable performance and flexible control.