The detection technique of pupil size has great significance in medical field. The algorithms for pupil size detection was studied by using image processing technology. By mkaing pre-processing on pupil images, acquiring the image processing area, edge detection, recognizing the pupil boundary with encoding chain and coding, and improving Hough circle to fit the pupil size, the detection on pupil size can be realized. Based on the detections on normal pupil images and diseased pupil images, the system can reach a pupil fitting coverage rate of 85%, and the detection only consumes a very short time, making it can be easily applied in practice.