Abstract: The press assembly of train wheelset is one of the important processes in the production of train wheels. The assembly has strict interference fit requirements, which requires real-time measurement of the outside diameter of the journal seat of the train axle. According to the measuring requirements of the outside diameter of the journal seat of the train axle, an automatic measuring system and method based on the principle of laser ranging are proposed in this paper. The horizontal angle and vertical angle of laser line are calibrated. The system can measure the axle in the range of ϕ150~ϕ205 mm, and the measuring accuracy is less than 10 μm, which overcomes the disadvantages of low efficiency and low accuracy of the traditional measuring method.