The present invention relates to a starting circuit which is characterized in that a power supply input end charges a first capacitor via a first resistor, when the voltage of the first capacitor exceeds a starting voltage of the starting circuit, a first power tube is conducted, a first transformer transmits the energy to a secondary side and an auxiliary winding, and a second capacitor starts tobe charged. Due to the consumption of a current in the circuit, the voltage of the first capacitor is reduced gradually, when the voltage of the first capacitor reduces to the starting voltage of a power supply detection module, and on the condition of not changing a peak current at the primary side of the first transformer, the discharge lower limit of a duty ratio adjusting module is changed, so that the demagnetization duty ratio at the secondary side of the first transformer is increased, and an output current of the first transformer and an additional charge current are increased, and accordingly, a maximum heavy load can be started normally.