Abstract: At present, the measurement of ultrasonic propagation direction by photoelastic equipment is mostly subjective observation of human eyes, the error is large, so it is difficult to achieve accurate quantitative measurement. By applying the Farneback optical flow method to photoelastic image processing, and using the optical flow diagram between frames to calculate the ultrasonic propagation direction in the photoelastic image, the dynamic sound field in the image can be captured. In the experiment, the ultrasonic waves in different propagation directions are measured with a maximal absolute deviation degree of 2.85°, and the measurement results are more accurate. Therefore, the Farneback optical flow method can be used to judge the direction of sound wave propagation in a photoelastic image with the characteristics of rapid, accurate and intuitive.