The measurement of fish caudal peduncle in aquaculture farm is of great significance in aquaculture management. At present, the main measuring method is depend on manual work which is inefficient and subjective. This study proposed a new measuring method based on image processing and linear fitting. Firstly, the images are collected in the fish farm using equipments; then, the external contour of the fish is obtained through edge detection and contour extraction based on the fish morphological character; finally, the length of caudal peduncle is figured out using corner detection and least square line fitting after the extracting of ROI of the caudal peduncle. The results show that the average measurement time is 1.01 seconds, the average measurement error is 0.34mm, and the accuracy of measurement has reached over 95%. Lots of studies have shown that the proposed method can not only solve the problem that manual operation is complex as well as subjective, but also avoid time-consuming measurement; and provide an effective way for non-contact mass measurement of fish caudal peduncle.