Aiming at the problems of low dissolved gas concentration and complex external interference signals in the existing power transformer oil, the monitoring system has great difficulties in small signal measurement and extraction.Based on the principle of photoacoustic spectroscopy, this paper proposes a technical scheme for online monitoring of gas in transformer oil.Through experiments, the data of dissolved gas in transformer oil at different voltages and at different times were analyzed and compared to determine the type of failure.Experimental results show that photoacoustic spectroscopy can quickly detect the faulty gas dissolved in transformer oil.The research provides reference and reference for the development of on-line monitoring technology for power transformers in China.