At present, when the oil-immersed transformer gas relay sends out the alarm signal, the oil chromatographic analysis takes a long time, the internal fault is easy to further develop, and manual sampling is dangerous, especially in recent years, a number of oil-immersed transformer fires and explosions have brought serious harm to personal, equipment and power grid safety. In this paper, by studying the new sensing technology of combustible gas in oil-immersed transformer, an on-line monitoring system of combustible gas in transformer gas relay is established, which can diagnose the internal fault of transformer timely and accurately. to improve the safety and reliability of transformer operation is of great significance to the safety and stability of person, equipment and power grid.