The invention provides a circuit and a method of a DEBUG card used for simulating 
server input 
voltage abnormality. Firstly, a PSU power supply is used to supply power to a 
server motherboard throughthe DEBUG card, and the 
motherboard works normally; then an output 
voltage value and duration of a 
DC SOURCE are adjusted, power supply 
mode switching is carried out on the two power supplies of the PSU and the 
DC SOURCE through a switch, and then input 
voltage of the 
motherboard is instantaneously changed; and a different shown phenomenon and BMC log of the motherboard for a different voltage value and different duration are compared with an original fault phenomenon and BMC log, and a fundamental cause of the motherboard work 
abnormality is positioned. According to the technical scheme of the invention, operations are simple, for different 
abnormality phenomena, the output voltage value is adjusted through the DEBUG card to reproduce the abnormality phenomena, fault positioning can be rapidly carried out, and problem analysis efficiency is greatly improved. The circuit and the method are suitable for use in input voltage abnormality investigation of all 
server motherboards on markets, realize accurate positioning for faults, and are high in generality.