The invention provides a power supply for a series compensation device and a control method for the power supply. The power supply comprises a current transformer, an isolation transformer, a rectification circuit, a supporting capacitor, and a digital-type DC conversion circuit. The control method comprises the steps: enabling the current transformer to be equivalent to an AC constant current source I, and enabling the isolation transformer to be equivalent to an exciting nonlinear inductor L0 and an ideal transformer T1, wherein the inductance L1 of the ideal transformer is the leaked inductance of the isolation transformer and the current transformer, the rectification circuit is simplified as a diode D1, the supporting capacitor is C0, and the digital-type DC conversion circuit and a load are equivalent to a resistor RL; maintaining the supporting capacitor in a stable section through controlling the magnetic saturation of the isolation transformer in a power equivalent model; enabling an isolation transformer magnetic circuit to enter a saturation state gradually when the supporting capacitor is charged to an upper limit value, and gradually reducing the secondary output of the isolation transformer to zero, thereby effectively achieving the control of the charging voltage of the supporting capacitor.