An automatic adjusting device is provided, which is used for adjusting an output power of a power supply and comprises an automatic adjusting circuit. The automatic adjusting circuit includes a comparing unit and a programmable signal generating unit. The comparing unit compares a limiting level and a protection level and produces a comparison signal. The protection level limits the output power provided by the power supply. The programmable signal generating unit generates the protection level and adjusts the protection level according to the comparison signal for adjusting the output power. The programmable signal generating unit will adjust the protection level according to the limiting level. Thereby, the output power can be adjusted automatically without manual adjustment. Consequently, the cost can be reduced and the adjusting accuracy can enhanced.