The invention discloses a device and a method for detecting mixed gas decomposition products of perfluoro-isobutyronitrile and carbon dioxide. The device comprises a gas decomposition module and a gasdetection module, wherein the gas decomposition module comprises an alternating current power source, a voltage regulator, a transformer, a resistor, a capacitor, a rectifier diode, a barometer, a closed cavity, a needle plate electrode, a gas valve and a micro-water generating device. The gas detection module mainly includes a gas bottle, a pressure reducing valve, a gas purifier, a GC-MC gas chromatography mass spectrometer, a gas collecting bag, an exhaust gas recovery device and a computer. During the stage of a discharge test, the closed cavity is filled with a small amount of water andoxygen and a proportion of mixed gas of the perfluoro-isobutyronitrile and carbon dioxide, the needle plate electrode of the cavity is subjected to pressure boosting by a high-voltage power source, after the corona discharge occurs, pressure boosting is stopped, and the voltage for 12 hours is maintained; and in the stage of gas detection, the gas collecting bag collects gas to be tested and the gas chromatography mass spectrometer is turned on, the gas in the gas collecting bag is injected into an instrument for detection, and the gas to be tested is subjected to the qualitative and quantitative treatment.