According to an actual engineering project and dynamic topographic features of moving workpieces, a new measuring model for large scale workpieces was established based on the virtual measurement datum structured by the two-dimensional laser triangle measuring method and the multi-sensor fusion rule. Then, a noncontact, high precision and low cost dynamic length measuring system based on the human-computer interaction was developed by using the error separation method to correct theoretical errors and motion errors. In the experiment, the two probe units were placed on the two sides of moving workpiece, the displacement between both end planes of workpiece and virtual measurement datum was measured simultaneously. Finally, A experimental measurement on the cylinder workpieces (1 000±25) mm at different speeds was performed, and results show that the system can offer the resolution less than 10 μm, measuring precision over 100 μm, and a maximum speed of 5 cm/s. According to the actual operation results, the proposed measurement techniques and measuring system can be well applied to the length detection for similar regular large scale workpieces in industrial production.