Abstract: The accuracy of image measurement is heavily dependent on sub-pixel edge detection.This paper analyzes the disadvantages of traditional edge detection method based on Zernike moments that is widely used now.And it also improves the performance in many fields，such as gray-value function modeling around the edge，sub-pixel localization at curved edge，real-time performance and so on.Then this paper proposes an improved sub-pixel edge localization algorithm based on rotate-invariant moments and uses this method to measure the thickness of cable sheath.The experimental results demonstrate that the proposed method is feasible and immensely improves the accuracy of localization.