A power supply device including a working power supply module with a plurality of power supply units; a plurality of monitoring units outputting a power failure signal when the corresponding power supply unit is in a power failure; a plurality of latching units respectively corresponding to the plurality of monitoring units one to one, receiving the power failure signal and a disabling signal, and outputting a latching signal when the power failure signal is received but the disabling signal is not received; and a positioning module used for receiving the latching signal and outputting a disabling signal when the positioning module receives a first latching signal output by one latching unit to disable other latching units, so as to enable the positioning module to position the latching unit which firstly outputs the latching signal to position the corresponding power supply unit which is firstly in a power failure.