Abstract: The presence of inclusions in the wind power spindle material 42CrMo4 steel leads to defects such as micro-cracks in the hot forming of large forgings. Different types of inclusions were studied to analyze their damage failure behaviour to the matrix in hot forming. The original as-cast 42CrMo4 steel was sampled, polished and corroded for SEM observation and EDS composition testing, nanoindentation experiments was carried out to obtain the real mechanical properties of the inclusions with large matrix damage. The results show that the as-cast 42CrMo4 steel has a coexistence of ferrite and pearlite at room temperature, with the presence of Al₂O₃, TiN, silicate inclusions and a small amount of CaS inclusions.The residual indentation depth of TiN is the smallest under the same load, indicating that it has the smallest plastic deformation and the best elastic recovery, followed by Al₂O₃.The matrix has the worst elastic properties at room temperature and its plastic deformation is the largest.Voids are observed in the vicinity of the inclusions, indicating that the inclusions are harder than the matrix and can disrupt the continuity of the matrix during the forming process, leading to cracks and other defects and thus affecting the quality of the product.