A two-stage switching power supply includes a first-stage power circuit, a second-stage power circuit, an output detecting circuit and a power control unit. The first-stage power circuit includes a first switching circuit. By conducting or shutting off the first switching circuit, an input voltage is converted into a bus voltage. The second-stage power circuit includes a second switching circuit. By conducting or shutting off the second switching circuit, the bus voltage is converted into an output voltage. The output detecting circuit generates an output detecting signal according to the output voltage. The power control unit controls operations of the first and second switching circuits according to the output detecting signal. A first-stage voltage gain value of the first-stage power circuit and a second-stage voltage gain value of the second-stage power circuit are altered with the output detecting signal, so that the output voltage is maintained at a rated value.