In the production of P92 seamless steel pipes by extrusion process, the refection wave intensive area for inner wall defect of seamless steel pipe was analyzed in many aspects, and based on macroscopic inspection, the surface and depth of the cracking areas were observed. Then, the microscopic fracture morphology characteristics were mastered by fracture scanning electron microscope analysis, and the non-metallic inclusions, cracks and matrix microstructure characteristics were clarified by metallographic inspection. Furthermore, the causes of cracks were analyzed, and it was found that a large amount of δ-ferrite phase was generated in the steel pipe during the normalizing process. When subjected to stress, the martensitic structure was brittle, and small cracks were easily generated in the δ-ferrite with poor plasticity. In particular, the ripening of M23C6 carbide around δ-ferrite reduced the grain boundary strength of the material. At the same time, the granular BN inclusions in the form of aggregates in the molten steel led to matrix discontinuity, which further aggravated the expansion of small cracks. Therefore, the heating temperature and time of the billet should to be controlled within a reasonable range during heat treatment.