The hot scratch of No.4 rack is the metal accumulation caused by the ploughing ditch. It belongs to the mechanical scratch in hot state caused by the scratch of roll and strip steel. The strategy to solve scratches is to reduce the rolling speed and reduction rate of No.4 stand, increase the temperature and concentration of emulsion, and reduce the heat of deformation in the metal zone. A rolling strategy based on the principle of reducing hot scratch was proposed. The reduction rate of No.4 stand was reduced from 34.1% to 29.8%, the reduction rate of No.5 stand was increased from 14.0% to 29.3%, and the rolling speed of No.4 stand was reduced from 1 615 to 1 272 m/min under the premise of keeping the exit speed of No.5 stand unchanged. Thus, the temperature of the rolling zone was reduced. At the same time, the lubrication principle with the concentration change of emulsifier as the core was put forward. By increasing the temperature of emulsion to 55-60 ℃ and the concentration of S3 to 5.5%-7.0%, the requirement of oil film thickness and stable lubrication in the rolling area and the requirement of forced cooling of rolls and strips outside the rolling area were taken into account. After taking the above measures, the incidence of hot scratch defect of the ultra-thin plate was reduced from 12% to 1%, the surface quality of the strip could meet the needs of high-end users.