Abstract: The length is one of main features of shrimps, which can be seen from the main skeleton line of the shrimp. So, it is very important to analyze the main skeleton line. The length of branches was studied based on a classical thinning method in this research. And the initial length was set as the threshold. The threshold should be corrected automatically after one branch was removed each time. This algorithm could remove branches of the skeleton efficiently and didn’t have an effect on the length of the main skeleton line. Actually, the length of skeleton was highly correlated with the real length, the maximum R2 was 0.946 and the minimum R2 was 0.747. The real length of another sample set was predicted with this method , and the mean relative error was 2.06%. Results demonstrated that this algorithm had prevalent applicability to distill the main skeleton line from the skeleton of the shrimp no matter what posture the shrimp’s body was. The main skeleton line produced by this algorithm not only had great smoothness and connectedness, but also kept one-pixel width. The geometric properties of the main skeleton line could be shown precisely.