Abstract: Online-measured length data of the steel tube is a piece of important information helpful for improving the quality and yield of the seamless steel tube production.It is rather difficult to perform online-length measurement of the seamless steel tube due to harsh working conditions of the stretch-reduced pipe production site such as high temperature,high speed,vibration,and dust.Studied here in the paper are the application of the machine vision method to the on-line length measurement of the stretch-reduced pipe,and the architecture and operation process of the on-line length measurement system,involving such aspects as establishment of the measurement coordinate system and determination of the photocapture timing and photographing control.Moreover the actual use of the system is described,and the stability of the system is analyzed.The actual application result shows that the system as designed has met the pre-set goal.