Ultrasonic detection of partial discharges in Gas-Insulated Switchgear (GIS) is mostly done with sensors mounted outside the equipment. Due to the complex internal structure of GIS and the strong sound absorption rate of SF6, it is difficult for traditional piezoelectric sensors to detect the weak partial discharge ultrasonic signal inside GIS. Therefore, based on the fully insulated and anti-electromagnetic interference characteristics of the fiber optic sensor, this paper proposes a built-in Michelson ultrasonic sensing technology, and designs a hollow mandrel structure to increase the sensitivity of the sensor. A GIS partial discharge detection test platform is built to carry out partial discharge detection tests. The results show that the built-in Michelson ultrasonic sensing fiber loop can detect the tip discharge of 8.8pC and the free particle discharge of 15.3pC after being arranged near the GIS partial discharge model.