The invention relates to a device for detecting a main transformer relief valve on line. The device comprises a main transformer and an oil conservator connected with the main transformer. The main transformer relief valve is arranged on the main transformer. The device is characterized in that a working solenoid valve is arranged on a port, connected with the main transformer, of the main transformer relief valve; a checking solenoid valve is arranged on the other port of the main transformer relief valve; the checking solenoid valve is connected with an air outlet of a pressure balance tank through a first connecting pipe; an air inlet of the pressure balance tank is connected to an air outlet of an air compressor through a second connecting pipe; the pressure balance tank is provided with a standard pressure gauge, a safety valve and a pressure release solenoid valve; and the working solenoid valve, the checking solenoid valve, the standard pressure gauge, the pressure release solenoid valve and the safety valve are respectively and electrically connected with a control module. The device has the advantages that the structure is simple, the main transformer relief valve can be inspected on line in time without being dismounted, and the safe operation of the main transformer is strongly ensured.