The chemical composition content, fracture feature morphology and metallographic structure of the early cracking failure piece for hot forging mold of NOS525 steel were detected and analyzed by direct reading spectrometer, digital Rockwell hardness tester, optical microscope, scanning electron microscope and energy spectrometer. The results show that the martensite needle in the microstructure is particularly slender, and the microstructure belongs to the ovrheated structure with coarse grains. The main reason is that the mold is heated for a long time in the high temperature region during the heat treatment process, and the structure grains grow up rapidly. As the carbon content in the matrix increases, the martensite morphology changes from strip to needle, the strength of the material decreases and the brittleness increases. Furthermore, by the detection of scanning electron microscopy, the cracked section of the mold is quasi-cleavage fracture and appears strip-shaped tearing edge, which indicates that the soft point zone with low hardness occurs in the microstructure, and the tearing edge feature of the fracture surface belongs to the unconverted retained austenitic organization after quenching. However, the retained austenite is prone to martensite transformation under stress to increase the structural stress and the risk of material cracking, and the usability of the mold is reduced. At the same time, during the use of the hot forging mold, there is a phenomenon that the working surface is not cooled in time or the cooling is not complete, so that the groove of the mold cavity has obvious thermal fatigue crack.