The invention discloses a hydrogen sensor for detecting dissolved gas in oil. The hydrogen sensor comprises a hydrogen-sensitive chip, a conversion circuit, a heater, a temperature sensor, an adaptorand a sealing ring, wherein the hydrogen-sensitive chip is mounted at the front end of a phi-9.5 hydrogen concentration testing probe based on a solid-state catalysis palladium-nickel alloy film technique, the hydrogen concentration testing probe and the hydrogen-sensitive chip are directly dipped into oil in application and are deeply put into screw holes of oil filling equipment, one ends of thehydrogen concentration testing probe and the hydrogen-sensitive chip are connected with a signal conditioning unit through a signal transmission line, one end of the adaptor is connected with an oilsampling hole of the oil filling equipment, the other end of the adaptor is connected with a base, and the sealing ring is used for sealing the adaptor with the oil filling equipment. According to thehydrogen sensor, the sensor probe can be directly put into transformer oil, a movable part is not adopted, and the sensor structure is simplified, so that the maintenance cost is greatly lowered, andthe service life of the hydrogen sensor is prolonged. Compared with a traditional offline monitoring measure and a traditional online oil chromatography, the hydrogen sensor has the advantages that an oil-gas separation film is not adopted, the hydrogen is not consumed, the selectivity is high, and the like.