Abstract: Spot defects appeared on the surface of a tinplate. The causes of spot defects were analyzed by macro analysis, micro analysis, salt spray accelerated corrosion analysis and porosity analysis. The results show that the tin sludge, air dust and organic matter residues in additive in flux in quenching tank adhered to the surface of the tinplate. The above substances were embedded in the coating under the effect of transitional boiling, absorbed water and oxidized, resulted in the formation of the spot defects on the surface of tinplate.