A modular power supply system incorporating a power status signal generator circuit to detect the voltage sag of the input peak voltage within a short period is proposed. The power status signal generator circuit mainly includes a peak detector that detects a peak value of an input voltage, wherein the peak detector comprises a first comparator that compares the input voltage with a predetermined reference voltage and sends an output signal according to a comparison result thereof, a RC network connected to the first comparator and is configured to charge and discharge the capacitor according to the output signal of the first comparator, and a second comparator connected to the resistor-capacitor network that compares a terminal voltage of the capacitor of the resistor-capacitor network with the predetermined reference voltage and generates a power status signal according to a comparison result thereof.