The invention describes a switching power supply system for automatically regulating circuit operating power and the method thereof. The switching power supply system has a sensing and monitoring unit connected to the computer system for detecting the operating status of the computer system and outputting a detection value, a setting unit for setting a trigger condition value, a storage unit connected to the setting unit for storing the trigger condition value, a comparator unit connected to the storage unit and the sensing and monitoring unit for comparing the detection value with the trigger condition value and outputting a comparison result signal, and a dual power system switching regulator unit connected to the comparator unit and to the converter through at least one dual power system unit for receiving the comparison result signal and regulating connection/disconnection between the dual power system unit and the converter based on the comparison result signal.