Abstract: Tarnishing reason of copper plate of VAMTOP casing couplings was analyzed by means of macro examination, microstructure examination, SEM analysis and X-ray energy spectrum analysis. The results show that the main reason for the tarnishing of the copper plating was that the surface copper oxidized and generated copper oxide. The dust color copper oxides hadn’t occupied the whole copper layer, and it wouldn’t destruct the combination of copper plate with matrix material. The tarnishing had no harmful effects on the service performance of the copper plate of the casing couplings.