To design an on-line diameter measurement system for Hot-rolled seamless steel tube production line.On one hand,it can play a stimulate part in the domestic pipe measuring technique.On the other hand,it can also make our domestic Hot-rolled seamless steel tube enterprises gain a strong product competitiveness with low input.Through the analysis of various detection methods and techniques contrast,this paper choose a CCD camera-based online caliper system design.The system mainly includes the hardware measurement portion and the image processing section,combining with software control technology and image processing technology,which can complete on-line measurement of heat tube diameter.Taking into account the complexity of the actual job site situation,it can choose a relatively simple and reasonable layout.The image processing section mainly to solve the camera calibration and the application of a function in Matlab,to achieve the diameter size display directly through the algorithm to calculate the image.I build a simulation platform in the design last phase,successfully,collect images for processing,to prove the feasibility and rationality of the design and make error in less than 2%.The design successfully using photoelectric detection technology to solve real work problems.