The invention discloses a test device used for testing insulating gas liquefaction temperature and a test method used for testing insulating gas liquefaction temperature. The test device comprises a constant temperature box, a constant volume bottle, a gas distributing device, a liquid phase detection device and a data monitoring and processing unit. The temperature range of the constant temperature box is adjustable. The constant volume bottle is arranged in the constant temperature box. The constant volume bottle is internally provided with a temperature meter and a pressure meter. The liquid phase detection device comprises an optical fiber scene processing unit and a schering bridge. The liquid phase detection device is connected and communicated with the constant volume bottle. The detection end of the schering bridge is arranged in the constant volume bottle. The gas distributing device is communicated with the constant volume bottle. The temperature meter, the pressure meter, the constant temperature box, the gas distributing device and the liquid phase detection device are connected with the data monitoring and processing unit through a signal line. According to the test device, the approximation problem existing on the basis of the ideal gas equation calculation value can be overcome and the insulating gas liquefaction temperature can be tested and verified on site; and two test methods of high operability are integrated to perform mutual verification of the test accuracy of the liquefaction temperature.