Abstract: Macro-examination and micro-examination of the characteristics of the cavity on the polished mold surface made of an advanced imported mirror plastic mould steel were carried out.The size and micro-hardness of the cavity were determined,and the X-ray mapping of main elements in the cavity were observed.Besides,the formation of the cavity was analyzed.The results show that the cavity on the mold surface is caused by the metallurgy defect,which is formed in the smelting and founding process of steel.The cavity bring down polishing quality and corrosion resistance property of the mold surface and led to early failure of the mold.The control measures of the metallurgy defect are proposed.