For the negative phase torque and rotational speed oscillation problems in the frequency conversion stage of the space voltage vector soft starter, the selection of the frequency conversion point was optimized according to the control method of the voltage vector. The initial value of the control angle of each frequency conversion stage was obtained with the motor load status, and the final value of the control angle was obtained with the constant voltagefrequency ratio principle, so as to optimize the control of the frequency conversion stage of the soft starter. Finally, the optimized soft starter was verified by simulation. The results showed that, when the soft starter was operating in the frequency conversion phase, the overshoot of the speed at the frequency conversion point was ameliorated, the current peak value was reduced, and no negative torque appeared.