Abstract: The welding joint of main steam pressure sampling tube of T91 steel in a power plant cracked during operation. The cracking reasons of the welded joint were analyzed by macroscopic observation, chemical composition analysis, hardness test, metallographic examination and pipe inspection. The results show that the cracking of the welded joint conformed to the characteristics of cold crack (delayed crack), the formation reason was that there was untempered martensite in the welded joint, the expansion of the pipe was hindered, and the welding residual stress was not eliminated, resulting in stress concentration. It was suggested that appropriate process measures and arrange the state of support and hanger reasonably to reduce the stress concentration of welded joints.