According to the years of accumulated data and practical experience,the optimum moisture of deep processing slag by vertical mill was 10%~13%, which has too much water, low yield, high energy consumption, and serious abrasion of vertical mill. When the water is too little, the material bed of vertical mill has instability, and vibration, which is not conducive to continuous production. In view of this situation, a water slag predewatering process and an optimization method for vertical mill efficient production are studied. The method has been proved by theory and experiment. The quality of product can be effectively stabilized with high feasibility and the cost is reduced by 1.6 yuan/t.