Abstract: Aiming at solving problems such as low accuracy of manual measurement for metro wheelset, a novel Online Measuring System based on laser displacement method was proposed to measure the parameters of wheelset. The article introduced the structure of the System, proposed the design of hardware and software, explained the measuring principles of the flange height and width as well as wheel diameter by using laser displacement sensor. The field experiments including wheelset test and train test were taken by using the System. The measurement results between the online system and workman were compared, and the result indicated that the proposed System with the advantage of accurate and good stability could be used to replace the existing manual measurement, and be meet the demand for measuring parameters of metro wheelset.