Abstract: To solve the problem of low sensitivity when using ultrasonic wave method to detect partial discharge (PD) in power equipment, a dielectric fiber FabryPerot (FP) acoustic sensor with quartz membrane was designed. For suitable frequency response characteristics of FP sensor, the natural frequency of FP sensor changed with diaphragm size was analyzed based on the theory of elasticity, and 3 FP sensors with different sizes were designed using ANSYS finite element analysis method. The ultrasound waves from 50 kHz to 250 kHz were generated from piezoelectric transducer (PZT) driven by signal generator in insulating oil tank. The ultrasound waves were detected by 3 FP sensors to get the amplitudefrequency characteristic curves. The experiment of plateplate PD detection by FP1~FP3 shows that the FP sensors with the first natural frequency of 150kHz could detect PD signals effectively, and the sensitivity can be significantly enhanced with the decreasing diaphragm thickness under the same natural frequency. The FP sensor sensitivity with the cavity length of 130 m is 2.47 times of that of the FP sensor with 100 m cavity length. The results show that the PD detection sensitivity of FP sensor can be enhanced by increasing the FP cavity length.