Using the multi-beam optical stress sensor (MOSS) to find the curvature of the film requires measuring the distance between the beams before and after reflection, and the radius of curvature of the film can be calculated based on the distance between adjacent incident (refracted) beams. The spacing of the beams can be determined by the center point of the received light spot. Extracting the center point of the light spot mainly uses Canny edge detection algorithm and ellipse fitting algorithm. The spot array image obtained with the CMOS sensor has a lot of salt and pepper noise mixed around the spot. For the traditional Canny algorithm, the edge of the spot cannot be accurately detected. An improved Canny edge detection algorithm is used, which can effectively eliminate the noise around the light spot and retain the edge of the light spot. Finally, an ellipse fitting algorithm is used to fit the edges to obtain all the center points of the spot array. At the end of the experiment, standard parts were selected for verification, and the radius of curvature calculated by the measured distance was compared with the actual one. The error result was 2% ~ 4%, which proved that the method was feasible.