Abstract: The cracking causes of a L360N failure bending pipe were analyzed by fracture anatomy analysis,metallographic structure analysis and physical and chemical performance detection. The results show that the structure of the hot-bending pipe is martensite and a small amount of bainite, which is inconsistent with the normalized structure（ferrite +pearlite） of L360N. Due to the brittle and hard characteristics of the martensite structure, the hardness, yield strength and tensile strength of the pipe are much higher than the standard requirements, and the elongation after fracture is lower than the standard requirements. This is the main reason for the failure of hot-bending pipe.