Abstract: The cold-rolled strip surface of Liuzhou iron and steel Co., Ltd. appeared blackening phenomenon in varying degrees. The causes of surface blackening of cold-rolled strip were analyzed by means of surface roughness analysis, micro morphology analysis and surface residual oxide analysis. The results show that the reason for blackening of the cold-rolled strip surface was the water corrosion during the pickling process, which resulted in a thin layer of iron oxide on the cold-rolled strip surface. After taking relevant measures to reduced the residual Cl^- mass concentration on the cold-rolled strip surface, the blackening degree on the surface of the cold-rolled strip was obviously reduced.