Abstract: A formula was proposed to describe the existence form of metal and CN^- ions in cyanide solution with various copper additions, metal ion concentrations and pH values. Based on the calculation results, the precipitation mechanisms of gold and silver were investigated when copper is added into cyanide solution containing gold and silver, and the causation was also discussed for that the cyanide consumption increased and the leaching rates of gold and silver decreased in condition of high copper content in the ore. When added into cyanide solution, the copper first reacted with free ions of cyanide and then "robbed" almost all of the CN^- from cyanide complexes of zinc, cadmium and nickel. CN^- was precipitated simultaneously as CuCN. The gold and silver complexes were "robbed" of CN^- and precipitated as AuCN and AgCN, respectively.