The invention relates to a soft start circuit comprising a soft start relay. The contact of the soft start relay is connected in parallel with a soft start resistor. The coil of the soft start relay is connected in a control loop. A time delay circuit is arranged in the control loop. When the control circuit outputs a control signal and delay time of the time delay circuit is up, the coil of the soft start relay is electrified. When the control signal is outputted, the soft start relay is not directly closed, and the soft start relay is enabled to be closed when the delay time is up; and output voltage is enabled to be stable due to existence of delay time so that zero-current closing of the soft start relay can be realized.