Abstract: The microstructure of the chip abnormal breaking area was studied by metallographic(OM), energy spectrum(EDS) and X-ray diffraction (XRD) analysis. The EDS component analysis and the XRD phase analysis of different microstructure were carried out. The results show that there is obviously metallographic abnormity in the chip abnormal breaking area, the abnormal microstructure is composed of tempered sorbite and white block structure. The energy spectrum analysis shows that the white block structure has the same composition as the matrix.The XRD analysis shows that the white block structure consists of γ-Fe and M/α-Fe.The comprehensive analysis shows that the 05Cr17Ni4Cu4Nb steel bar has gone through the local abnormal heating process after the final heat treatment, and the material is locally austenitized and partially converted to a martensitic structure during a subsequent cooling process. Due to the low plasticity of the structure, chip abnormal breaking occurs during the turning process.