In order to monitor the working condition of the power system more accurately, a discrete multi-point detection scheme applied to partial discharge (PD) detection of power transformers was proposed. Based on the heterodyne interference principle of optical fiber, the sensing system was composed of four sensing heads to realize multi-point detection and partial discharge source localization. The sensitivity of the sensor was improved by optimizing the structure of the sensor head. At the same time, the compact package of the sensor enabled it to be installed in the power transformer, which improved the accuracy of the detection results. The experimental results show that the single-channel resolution of the proposed sensing scheme can reach 4 mrad. The constructed three-phase power transformer model is experimentally demonstrated and analyzed, and the resolution of sound source localization can be 1 cm when the 150 kHz PD ultrasonic multi-channel measurement is carried out. Because of its compact structure, high resolution and low cost, the proposed scheme has great application potential in security detection of power system.