The invention discloses a dissolved hydrogen limited value sensor in power transformer oil and a detection system. The dissolved hydrogen limited value sensor comprises an optical fiber, wherein a grating is engraved on a fiber core of the optical fiber; a region, which corresponds to the grating, on the optical fiber is an optical fiber grating region; one side surface of a wrapping layer of the optical fiber grating region is polished into a planar polished surface; a hydrogen sensitive metal layer is attached to the polished surface. The limited value sensor is high in sensitivity and high in timeliness; once the concentration of dissolved hydrogen reaches a limited value, the wavelength change amount quickly reaches the limited value, so that a worker can timely and effectively find a fault to prevent further development of the fault. By adoption of an optical signal, the influence on a measurement result due to a complicated environment in a transformer can be avoided, the interference resistance is high, a detection signal is a wavelength, and the influence caused bv the factors such as the intensity of a light source signal, aging of a detector and connection loss is avoided. The dissolved hydrogen limited value sensor is simple in structure and is prepared based on the optical fiber; the diameter is small, and the limited value sensor is easily arranged in the transformer oil and convenient for detection; furthermore, normal operation of the transformer cannot be affected, and the limited value sensor is suitable for on-site application of the transformer.