The present relates to a start-up circuit, which is used for starting up a variable power supply circuit, which comprises a detection circuit and a transition circuit. The detection circuit is used for detecting an output voltage of the variable power supply and producing a detection signal. The transition circuit is coupled to the detection circuit. It transits the level of the detection signal and produces a control signal for starting up or cutting off the variable power supply. Thereby, the problem of incapability in transition can be avoided as well as achieving the purpose of low power consumption.