Abstract: As H13 steel moulds always suffer early softening, deformation, cracking and wear during the production of aluminium profiles, chemical composition analysis, hardness testing and microstructure observation were conducted to reveal the failure causes. The results show that the quenching and tempering process of the mould before nitriding is unreasonable. There are a small amount of inclusions and undissolved carbides, which make the overall toughness low. Moreover, The nitride layer is thin, and the hardness is low, and its microstructure is unstable. The bonding force between the nitrided layer and the substrate is poor, which aggravates the spalling and wear of the nitrided layer of the mould. It is suggested to optimize the heat treatment process and nitriding process of the mould to improve its hardness and impact toughness, thereby prolonging the service life of the mould.