The cracking reason of atmospheric corrosion resistant steel B480 was investigated by using macroscopic observation、metallographic technology、energy spectrum analysis and EBSD technology. The results show that in the middle of the thickness of the steel there exists some ferrite streaky structure, which has orientations of <111>, <001>//ND.The segregation of element P was found in these ferrite streaky structure and its percentage composition was quite above the normal level.The rising hardness and the brittleness of the ferrite streaky structure were caused by the reasons mentioned above eventually let cracking event happen because of the deforming inharmony.