The detection and capture of partial discharge (PD) signals are crucial for sensing and assessing the insulation state of power equipment, as well as for fault warning and diagnosis. An ultrasonic sensing technology based on Sagnac unbalanced interference (SUI) is proposed to effectively detect and capture PD acoustic emission signals from power equipment. The structure of the SUI optical path and the process of optical path interference are analyzed. It clarifies the mechanism by which the SUI perceives external ultrasound and establishes the sensing equations for acoustic waves. In order to meet the demand for detecting weak ultrasound signals in power equipment, the research also analyzes the influence of SUI optical path structure parameters on ultrasonic sensing characteristics and proposes optimal structural design parameters for the sensor. Experimental test results demonstrate that the proposed SUI system can detect ultrasound in the frequency range of 30 kHz to 150 kHz, exhibiting wide frequency coverage and high sensitivity. Moreover, it successfully captures the acoustic emission signals of needle plate electrode discharge. The proposed SUI shows the advantages of low cost, simple and stable structure, which paves the way for the optical detection of partial discharges in power equipment.