Through analyzing the problem of emulsion ingress into backup roll thin oil lubrication system in cold rolling,a solution was presented,including improving the sealing structure and selecting the lubricating oil with high water separation property.By improving the sealing structure,the lubrication system can prevent emulsion ingress into thin oil,and by selecting the lubricating oil with high water separation property,the lubrication system has a high ability in response to sudden accident of emulsion ingress.The improved method was applied successfully in the pickling tandem cold mills.