The present disclosure provides a switch power supply circuit, which includes a step-down transistor, a control module, a first switch transistor, a second switch transistor, a third switch transistor, and a power switch transistor. In the switch power supply circuit of the present disclosure, the first terminal of the second switch transistor is directly coupled with the input voltage, and the second switch transistor can directly obtain sufficient driving current from the input voltage to drive the power switch transistor. The present application can ensure the reliability and efficiency of the system without an additional energy storage capacitor, which reduces the costs of the system.