An automatic measuring system of locomotive wheelbase of bogie was designed, which was based on the configuration software WINCC of the German firm, Siemens. In this new system, the relative magnetic scale was used as a sensor and PLC highspeed pulse counter was utilized to acquire and calculate data. The wheelbase measurement is accurate to ±01 mm through experiment, which provides a basis for the assemble standard of locomotive bogie in the process of railway vehicle assembly.