A novel rapid on-machine length measuring system based on 3D laser scanning was developed for mobile and large scale cylinder workpieces. By using the outstanding characters of laser scanners on fast continuously acquiring the mass of measurement points in a very short time, the multi-sensor fusion technique and a 2D error separation method were respectively introduced in this system in a virtual environment. Virtual measurement datum planes were established based on virtual registration sites, which followed the variant surface of measurement target. Then, the 3D displacement field measurements between end points and virtual measurement datum planes were built. A similar cylinder workpiece measured by Coordinate Measuring Machine (CMM ) was applied to calibration of the system. Finally, the experiments on the cylinder workpieces (1 000±25) mm with different diameters were carried out to determine the accuracy and reliability of measuring system. The experimental results indicate that a workpiece with a diameter about 50 mm is perfectly measured in only 1 s with the resolution less than 0.010 mm and the measuring precision over 0.050 mm. According to the actual operation results, this proposed measuring system is characterized by high automation and high efficiency, and can be well acceptable for application as online length detection for large scale workpieces in industrial production.