A power supply system and method includes a power grid input unit and a diesel generator input unit, separately used for supplying an alternating current to a power supply unit. An automatic transfer switch unit is connected to the power grid input unit and the power supply unit or connected to the diesel generator input unit and the power supply unit, which is used for converting the received alternating current into a direct current. A control unit, which is used for monitoring a current load current and current diesel generator power, determines when to turn off a preset number of power supply loads according to a magnitude relationship between the current diesel generator power and the current load power, as well as according to priority levels of current loads.