Butt flash welding experiment is carried out to solve the UT failure problem of U71 Mn steel rail. The appearance of welded joint and the characteristics of defects are analyzed. Most defects lie near the fusion line mainly in the forms of holes and cracks,and sulfides are found on a few hole surfaces and crack tips. These hole defects are caused by the overburning produced by improper welding parameter matching. The overturning makes the low-melting-point segregation grain boundary melted and separated. By adjusting welding parameters, the overburning is eliminated and the UT qualification of joints is ensured.