A testing device has been designed for measuring the straightness of welded joints in rails. Machine movement control is realized by using PLC contr-oller,the multi-function data acquisition card was used to collect data,and the Labview software program was utilized to realize processing,displaying and storage of measurement data. Through a large number of measurement experiments,we confirm that the mechanical design of the testing machine can meet the requirements of long-term measurement reliability because of its high repeatability;At the same time,the result of contrasting the Germany SEC electronic measurement data curve and the straightness degree shows that system measuring accuracy of the testing machine is very high,and it can satisfy the requirements of measuring standards for straightness of welded joints in rails. This research will provide the reference for developing promotion stationary straightness measuring machine in the future.