Effects of coal metakaolin （CMK） on corrosion resistance of cement mortar and its internal steel bar under chloride and sulfate attack were characterized by electrochemistry， flexural strength test， mercury injection and X-ray diffraction. The results show that CMK cement mortar cover can reduce the corrosion degree and corrosion rate of steel bars， delay the time of pitting damage and improve the corrosion resistance of steel bar. The addition of CMK reduces the expansion ratio and the flexural strength loss of cement mortar during the process of chloride and sulfate attack. CMK can refine the pore structure of cement mortar， inhibit the formation of gypsum， ettringite and other erosion products in cement paste.