The traditional material used as a secondary support for mounting Chinese paintings was Xuan paper. In order to achieve a uniform mounting, the backing papers should have minimal expansion and shrinkage potential. The regularity of dimensional change of Xuan paper was studied, and the reasons for dimensional unstability are discussed. The results showed that the paper had the characteristics of dimensional unstability, and the extent of deformation was related to the component and drying methods of paper. The dimensional stability of paper could not be improved through the gradual drying method. With the circling times of drying and wetting increasing, the extent of deformation gradually reduced. To diminish the deformation of paper, some pre-treatment could be done. -- ICCROM