Abstract: By means of chemical composition analysis, macro examination, metallographic examination and so on, the failure reason of QDH concave mould was analyzed. The results showed that decarburizing layer of the concave mould wasn′t removed absolutely, the surface of mould was coarse, nervation and wave nitride was came into being in the nitrided case. All of these made the mould impact toughness significantly reduced, leading to mould happened early cracking, collapse and local spalling.