Doppler frequency shift in optical fiber is measured by fiber frequency shift interferometry, and a method of optical fiber ultrasonic sensing is obtained. The optical fiber loop wound on the piezoelectric ceramic is inserted into the frequencyshift interference Sagnac interferometer, where the piezoelectric ceramic is taken as the ultrasonic signal source, the acoustooptic modulator is adjusted so that the interference signal is biased at the zero point to reach the highest sensitivity of the system. Through the frequency and amplitude of the interference signal, the Doppler frequency shift in the fiber loop caused by ultrasound is measured, and the ultrasonic signal acting on the fiber loop is obtained. The experimental results show that, the relative error of the sensing method is 0.001% for the ultrasonic frequency measurement, and the frequency response has a good linearity in the measured range of 20 kHz to 200 kHz. There are some application prospects of the proposed method in the aspects of pipeline health monitoring, solid internal crack monitoring and structural damage monitoring of large machinery and equipment.