The present disclosure relates to the field of circuit design. The embodiments of the present disclosure provide a soft switch circuit, a circuit board assembly, and a switch power supply. The soft switch circuit includes a power supply, a first switch transistor, a second switch transistor, a third switch transistor, a fourth switch transistor, a capacitor, a main inductor, an auxiliary switch transistor, an auxiliary inductor, and a control unit, wherein the control unit is configured to: if the current of the main inductor is positive, turn on the auxiliary switch transistor before the first switch transistor and the fourth switch transistor are turned on; and if the current of the main inductor is negative, turn on the auxiliary switch transistor before the second switch transistor and the third switch transistor are turned on.