An automatic light regulating power supply device, connected between an AC power supply and a light-emitting-diode (LED) light string, to drive said LED light string to emit light, comprising: a sensing-and-setting unit, and a digital power control unit. Wherein, the sensing-and-setting unit is used to sense infrared signals, and generate a plurality of brightness selection signal. An AC/DC conversion circuit is provided in the digital power control unit, to convert the received AC power into DC power and output the DC power. The digital power control unit is connected to the sensing-and-setting unit, and regulates its power output based on infrared signals of the sensing-and-setting unit and the brightness selection signals, to vary illumination of the LED light string, in achieving energy conservation and lower power consumption.