The invention belongs to the technical field of detection of electric equipment and particularly relates to a fiber bragg grating hydrogen sensor used for detecting hydrogen in transformer oil. A side edge polished fiber bragg grating is used as a substrate, and a palladium-silver alloy and a pure metal palladium film are used as hydrogen-sensitive thin films; hydrogen concentration of the environment is converted into stress by using a principle that the hydrogen-sensitive thin films are reacted with the hydrogen and the hydrogen absorption volume is expanded; the hydrogen-sensitive thin films are tightly combined with the substrate so that the strain generated on the hydrogen-sensitive thin films is conducted to the substrate to be converted into an offset of a central wavelength; and the hydrogen concentration value in the environment can be detected through detecting the offset of the central wavelength or the amplitude of transmitted beam intensity. The fiber bragg grating hydrogen sensor has the advantages that an optical method is adopted so that the fiber bragg grating hydrogen sensor is suitable for a complicated electromagnetic environment in a transformer and is not influenced by electromagnetic interferences, and the measurement precision is improved; the electromagnetic interference is safe and reliable and can be directly arranged in the transformer oil; and oil-gas separation is not needed so that the real-time monitoring can be realized and the monitoring precision is improved.