The invention belongs to the technical field of detection of power equipment and particularly relates to a system for detecting local discharge in transformer oil on the basis of a fiber Bragg grating. The system comprises a fiber Bragg grating sensor, a laser source, an optical coupler, a photoelectric detector, a digital waveform recording system, an ambient temperature sensor and a feedback circuit, all distributed in a cavity of an oil-immersed power transformer. The laser source outputs light of specific wavelength; the light is transmitted to the fiber Bragg grating sensor through the optical coupler; the fiber Bragg grating sensor converts an ultrasonic signal about local discharge of the transformer into reflected light of a light intensity signal, transmitted to the photoelectric detector; the digital waveform recording system records an electrical signal output by the photoelectric detector; the ambient temperature sensor measures ambient temperature, a feedback circuit changes output wavelength of the laser source, and thus the output wavelength is kept equal to operating wavelength of the fiber Bragg grating sensor. The system is good in insulation, capable of adapting to complex conditions and high in measuring sensitivity.