The invention discloses an optical fiber ultrasonic detection system and detection method with self-adaptive transformer partial discharge. The system includes: light emitted by a C-waveband adjustable laser light source enters an optical fiber acoustic emission sensor and is split into two beams which enter first and second photoelectric conversion circuits respectively, respective corresponding analog voltage signals are obtained after processing, an analog-to-digital conversion circuit processes the analog voltage signals and obtains digital voltage signals, a single-chip microcomputer carries out subtraction on the digital voltage signals, a difference value is compared with a set voltage range, and the difference value of the digital voltage signals is guaranteed in the set voltage range; and the processed signals are transmitted to a differential amplification circuit which processes the signals and sends the signals to an active filter circuit for output. According to different splitting ratios of different sensors and different light path loss, central wavelength of the laser light source is automatically adjusted, so that the whole measuring system has optimal response and dynamic range, and a demodulation circuit maintains a consistent amplification factor sum, facilitating subsequent calibration and quantification work.