The paper aims to discuss the effects and the mechanism of the application of Al2(SO4)3 on strength and water resistance of corrugated paper in surface sizing process to provide theoretical support for better application of Al2(SO4)3 and improvement of paper performance. The effect of Al2(SO4)3 on the pH value and viscosity of starch paste and on the strength and water resistance of paper were firstly investigated through experiments. Then the mechanism of Al2(SO4)3 affecting paper sizing was discussed by DSC and SEM observation for starch film. Addition of Al2(SO4)3 can significantly reduce the pH value and viscosity of starch paste; in a certain range of dosage, the strength of paper could be significantly improved by the addition of Al2(SO4)3, the optimal dosage was about 6% (mass fraction); the water resistance of the paper would firstly be increased and then decreased with the adding of Al2(SO4)3, and the optimal dosage was about 3% (mass fraction); a layer of nano-crystallizations was found on the surface of starch film containing Al2(SO4)3 by SEM observation; DSC detection also showed that the glass transition temperature of the starch film was significantly improved by the adding of Al2(SO4)3. The nano Al2(SO4)3 crystallizations layer on the surface of starch film and the increase of its glass transition temperature are both the important reasons for the Al2(SO4)3 to affect the sizing effect of paper, especially the strength performance.