Aiming at the detection requirements for ultrasonic waves resulted from partial discharges in power transformers, a fiber-optic ultrasonic wave sensor system with a simple and novel structure utilizing the self-mixing interference effect in a semiconductor laser combined with Sagnac interferometric techniques is proposed. Under the condition of weak external optical feedback, the partial discharges can be determined by measuring the changes of oscillating frequency of the laser diode induced by the disturbances of sensing fiber by discharge-induced ultrasonic waves with the Sagnac interferometer. The operation principles of the system are explored and experimental researches are carried out. The ultrasonic waves produced by various ways such as pencil broken, metal pin free falling and piezoelectric ceramics are detected. The sensing performances of the experimental system are tested. The discharge phenomena in a power system are simulated with high-voltage discharges occurring between two parallel copper wires, including the corona discharges and partial discharges. They are detected under different humidity levels and voltages. The satisfied results are obtained. All the experimental results show that the proposed sensor system has high sensitivities and good high frequency responses as well as a capacity to detect weak ultrasonic waves up to 300 kHz. It can be concluded that the proposed system can meet the requirements for partial discharge detection in the power industry.