The invention provides a power supply protection device, a power supply device, a switch fault diagnosis method and a recording medium. The power supply protection device obtains an open voltage whenthe second switch is in a closed state and the first switch is in an open state when the power supply is discharged, and obtains a closed voltage when the second switch is in a closed state and the first switch is in a closed state when the power supply is discharged. Whether a first switch has failed is diagnosed on the basis of an on-voltage and an off-voltage, and whether the first switch has failed is diagnosed on the basis of a comparison result obtained by comparing a differential voltage between the off-voltage and the on-voltage with a third threshold voltage corresponding to the reference voltage.