A powering circuit of an AC-DC converter, for converting a high AC input voltage into a low DC output voltage to provide a load voltage in a stable DC bias range, includes a rectifier, a sensing circuit, a control switching circuit, and a voltage regulating capacitor. The rectifier has a primary side coupled to an AC power supply and a secondary side for outputting a DC power supply. The sensing circuit compares the AC input voltage with a preset reference voltage, and turns on a second switch in the control switching circuit when the AC input voltage is lower than the reference voltage, thereby providing a low DC output voltage. The control switching circuit sustains the DC output voltage in a stable DC bias range. Therefore, in addition to reducing the power consumption of the second switch, this circuit structure is simple and can achieve the purpose of circuit integration.