Abstract: The bright plating tin coating on the violet copper capillary is subject to corrosion and discoloration. Corrosion will inevitably result in degrandation of decoration and solderability. The causes of discoloration are described in detail, we will find when adding 3~5g/L Bi₂(SO₄)₃ to ordinary bright plating tin liquor, the 4-5 μm bright plating tin coating against discoloration is formed on the violet copper capillary under 3~4A/dm² of current density.