This application provides a switch power supply circuit and a terminal device, and the switch power supply circuit uses a second switch transistor to replace a diode in an asynchronous switch power supply circuit. A turn-on impedance of the switch transistor is very small, only tens of milliohms, so that a turn-on voltage drop in the switch power supply circuit is reduced from hundreds of millivolts to tens of millivolts, power loss of the switch power supply circuit is greatly reduced, and output efficiency is improved. Moreover, this solution uses a discrete component to form a control circuit to control a switch status of the second switch transistor without additionally setting a pulse width modulation (PWM) controller, and costs of the discrete component are far lower than costs of the PWM controller, reducing hardware costs of a switch power supply and simplifying a circuit design.