The embodiment of the invention discloses a GIS insulation degradation detection device and method and a GIS insulation degradation diagnosis system. The GIS insulation degradation detection device comprises a voltage regulation module, a discharge gas chamber, a sampling module, an oscilloscope and a mass spectrometer. An input end of the voltage regulation module is used for inputting alternating current, and an output end of the voltage regulation module outputs adjustable voltage. A discharge electrode is arranged in the discharge gas chamber and is electrically connected with the output end of the voltage regulation module. An input end of the sampling module is electrically connected with the discharge electrode, and an output end of the sampling module is grounded. The oscilloscopeis electrically connected with the sampling module and is used for monitoring the discharge capacity of the discharge air chamber. A sampling hole is formed in a wall of the discharge gas chamber, andthe mass spectrometer detects gas components in the discharge gas chamber through the sampling hole. According to the embodiment of the invention, the internal defect type of GIS equipment and the severity degree of insulation degradation can be accurately identified, and the collected data can form a diagnostic decision tree conveniently.