An apparatus for a power supply with brownout protection and the method thereof are proposed. As a brownout condition occurs, the apparatus informs at least one microprocessor to process the shutdown process timely. The apparatus includes a power factor correction (PFC) circuit, and a bus capacitor generating a bus voltage by receiving an input voltage via the PFC circuit, and a PWM circuit generating multiple output voltages for the microprocessor, and a supervisor circuit generating a power-good signal in response to a proportional bus voltage, and a brownout circuit for disabling the boosting operation of the PFC circuit under a brownout condition. When the boosting operation is disabled, the bus voltage starts to fall. As the proportional bus voltage is lower than a threshold voltage, the supervisor circuit generates the power-good signal. Therefore, the microprocessor can process the shutdown procedure timely.