In order to improve the accuracy of dissolved gas analysis(DGA) in transformer oil, reduce manual operation and make full use of the current test equipment, a kind of automatic degassing unit was established in this research and combined with the gas chromatograph in laboratory to realize the automatic degassing and sample analysis of the characteristic gases in transformer oil. The tests of detection time, sensitivity, accuracy and repeatability show that the the automatic degassing unit meets the requirements and can effectively improve the detection efficiency.