Partial discharge (PD) usually occurs before the insulation failure of gas-insulated-switchgear (GIS). Early failures in GIS can be detected effectively by monitoring the vibration signal initiated by PD. In this paper, a new type of acoustic-emission (AE) sensor based on PANDA polarization maintaining fiber (PMF) is proposed to detect the PD-induced ultrasound information accurately in GIS. Pressure load affects the birefringence of the PMF, leading to variation in light output intensity. Simulation and experimental results indicate that the effective refractive index difference between the x and y polarization modes has a linear relationship with the external pressure. Therefore, pressure variations generated by ultrasound can be obtained by interference demodulation between the x and y polarization modes. The proposed sensor has many advantages such as simple structure, small volume, and strong environmental tolerance. The results of this work provides a new direction for more effective PD detection methods and fault diagnosis approaches.