Abstract: The weld seam crack of P91 steel pipe and F91 steel pipe of a 600 MW supercritical unit appeared. The causes of weld seam cracking were analyzed by means of macro observation, chemical composition analysis, metallographic examination, hardness test, fracture surface micro morphology analysis and so on. The results show that a large number of creep cavities produced in the fine grain zone of the pipe weld seam after long-term high temperature operation. The cavities aggregated into hole-chain and formed micro cracks. Finally, the type TV creep cracking occurred in the pipe weld seam.