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A high-speed length measuring system for mobile and large-scale cylinder workpiece was designed and constructed. This measuring system is characterized by two stationary laser scanning systems placed on two sides of workpiece responsible for the acquisition of points of key parts. Meanwhile, for making the measuring process faster and more flexible, the virtual measurement datum planes play an important role, and are step-by-step generated according to various features of measured surface in an online virtual inspection environment. Moreover, modeling and identification of end point of workpiece are established based on height variations in its nearest neighborhood. The three-dimensional dynamic length measurement data are then obtained by calculating the displacement between end point and corresponding datum plane. The prototypal system has been tested on different real cases with several typical cylinder workpieces (length 1000 ± 20 mm, diameter 50 ± 25 mm). Compared with the measurement results of coordinate measuring machine, the root-mean-square errors of the measurement data are smaller than 0.035 mm. The system measurement uncertainty is better than ±0.040 mm. The measuring time is less than 2 s. The proposed measuring system features high automation and high efficiency, and is most promising for on-machine applications in large geometric dimensional measurement.