Partial discharge (PD) ultrasonic detection based on optical interference technique shows practical advantages such as anti-electromagnetic interference, intrinsic safety, and implantability of power equipment, but the sensitivity improvement is still a necessity. In this paper, the sensitivity is improved by polarization state control and interference structure parameter adjustment. For the disorder of polarization state in the interference system, the detection sensitivity within 20-300 kHz can be improved by 6.48 dB with the help of polarization control. In order to meet the PD detection requirements for various power apparatus, an optical PD detection method based on Sagnac interference structure is proposed. The frequency response can be dynamically adjusted according to the type of PD signals. For the typical PD model in the air, the frequency response of the sensor can be controlled at 20-100 kHz by using a 2000 m delay fiber, and the detection amplitude of the sensor is 5.85 times than that of the conventional PZT sensor. For the detection of PD signals in insulating oil, the frequency response can be moved to 20-160 kHz by using a delay fiber with length of 500 m, and the detection amplitude of the sensor is 12.68 times than that of PZT sensor. Therefore, the proposed PD sensor based on the Sagnac interference shows the advantages of adjustable frequency response, multi-scenario application, and high detection sensitivity.