A stray gassing test device for transformer oil under condition of coincident electric field and temperature was developed and the corresponding test method was established independently. Influences of oxygen、temperature、electric field strength、metal deactivator content on the transformer oil stray gassing were studied, and the reason for hydrogen overproof was analysed. The results showed that within the range of studied electric field strength, temperature has greater influence on the oil gassing characteristics than electric intensity. Hydrogen overproof of transformer oil was mainly caused by triazole derivatives and benzotriazole derivatives oxidation. This type of metal deactivators contained unstable N-C-N and –CH2-. Besides, stray gassing of 7 transformer oils with other triazole or benzotriazole derivatives were tested to confirm the stray gassing mechanism.The conclusion was very useful for transformer oil product R&D and application in electrical equipment .