Conventional acousto-ultrasonic piezoelectric sensors can be easily interfered in strong electromagnetic environments such as partial discharge (PD). Current research shows that the effective refractive index difference between two orthogonal polarization modes increases differently with external pressure. Light output intensity can be detected by interference demodulation and depolarization from the polarization-maintaining fiber (PMF) between the two orthogonal polarization modes, and then pressure variations can be obtained. In this study, a new type of acousto-ultrasonic sensor based on the principle of polarization interference is designed to detect PD-induced ultrasonic signals. Simulation and experimental results are also discussed by comparing the following two types of PMFs as the sensing unit: polarization-maintaining photonic crystal fiber (PM-PCF) and polarization-maintaining and absorption-reducing (PANDA) fiber. Analysis results have indicated that the PM-PCF has a higher sensitivity towards external pressure, because its birefringence has a linear relationship of 2.02 × 10−6 RIU/MPa, far beyond 3.01 × 10−11 RIU/MPa of the PANDA fiber. Then, the function between external pressure and light output intensity can be analyzed. The proposed fiber optic sensor using the PM-PCF as the sensing unit has a higher signal-to-noise ratio (SNR). Therefore, this type of sensor can more easily find the initial failure of equipment.