In order to reveal the scientific connotation of alum gelatin solution who is widely used in curing Xuan paper, the solidification and ink droplets shading experiment were performed to investigate the roles of alum; the effect and synergistic reaction of glue and alum were studied by scanning electron microscope (SEM), highfield ²⁷AlNMR spectroscopy (²⁷AlNMR) and attenuated total reflection infrared spectroscopy (ATRFTIR); the negative impact of alum was discussed through damp and heat aging test. The results showed that:1) As a drypromoter, alum could accelerate the glue agglomeration and solidification; as a softener, alum could increase glue elasticity. 2) Alum could also be used as retention aid, played the role of ink droplet retention. When paper treated by alum gelatin solution, the performance of antiink droplets scattering was better than pure glue or alum; If only existed glue without alum, the ink would float on paper and it was easy to leak ink for numbers of pores in paper.3) SEM, ²⁷AlNMR and ATRFTIR results showed that the gum and alum had synergistic reaction in improving hydrophobicity and ink droplets scattering property. Form of Al(H₂O)³⁺₆, AlSO⁺₄ and Al₃₀([Al₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺, alum bonded together with hydroxyl or carboxyl from gelatin. The tight network structure they formed could improve the crosslinking and waterresistance of glue. Meanwhile, as a bridging role, Al ions could transform negative gelatin particle into positive charge, prompt gelatin particles deposited on the negatively charged fibers. 4) The greater the amount of alum, the stronger the acid, the paper was more brittle after ageing. So it was a serious threat to the life of paper. In summary, the study showed that the traditional alum gelatin solution played the role of drypromoter, softener and retention aid, but larger amount of alum made paper serious acidification. At last, it is suggested to develop a new kind of sizing agent to replace alum gelatin solution used in prior process.