Image measurement technique has been a new high-performance measurement technology in recent years, it occupies an increasingly important position in the measurement field. It has been widely used in geometry measurement field because of its advantage of non-contact,Shigh accuracy,Shigh speed. The paper puts forward an improved Sobel edge detection algorithm and proposed a new sub-pixel edge detection algorithm based on the Gaussian-Hermite moments. Then apply it to the cable sheath material thickness measurement and achieved a precise and fast measurement system.