In this paper, a fiber optic sensing system based on fiber optic Sagnac interferometer is proposed for ultrasonic detection of multi-point partial discharge. The detection system consists of a light source, a single-mode fiber delay line, a fiber coupler, a phase modulator, a polarization controller and a fiber Sagnac interferometer with a contrast ratio of 1. Firstly, the principle of ultrasonic detection of the sensor system is introduced, and then an ultrasonic experimental platform for simulating partial discharge is built. Experiments on the propagation of partial discharge ultrasound in steel plates and transformer insulating oil using surface waves excited by irradiation of steel plates with a Nd∶YAG laser and a high-frequency acoustic source. The experimental results prove the effectiveness of using this system for partial discharge ultrasonic detection, with a signal-to-noise ratio of more than 40dB in the frequency range of 100 kHz-200 kHz. At the same time, the experiments validate the multi-point detection ability of the Sagnac sensor for partial discharge ultrasonic. The optical fiber ultrasonic detection system has a simple structure, high frequency response and low cost, and has great potential in the fields of partial discharge.