The invention discloses a power supply detector for detecting a power supply plug-in of a microcomputer protective device. The power supply detector comprises an access terminal row connected with the power supply plug-in to be detected, the access terminal row at least comprises four groups of electrical connection terminals respectively corresponding to a chip work power supply, a sampling power supply positive bias, a sampling power supply negative bias and a relay driving power supply, each group of electrical connection terminals are correspondingly connected with one on-load detection loop, each on-load detection loop is internally provided with a corresponding load resistor in a series mode, and the two ends of each load resistor are connected in parallel with one display device for displaying the output voltage of the corresponding power supply plug-in to be detected. The power supply detector is internally connected with the load resistors according to the requirement for each power supply output power, and the on-load capability of the power supply plug-in can be detected. The detector is portable and small, can be conveniently carried and used by after-sale service personnel and user relay protection personnel, is simple and convenient to operate and visual in test, and is capable of rapid detection of the performance of the power supply plug-in.