The HDI trimer is a chemical reinforcement material for paper. In order to evaluate its safety for the reinforcement of handwriting on paper relics, we selected handwriting materials, Chinese ink, carbon ink, blue-black ink, pure blue ink, red ink and pencil, which are commonly seen in paper files. We noted the color changes before and after reinforcement and the diffusion of the handwriting materials. We also compared the HDI trimer with other reinforcement materials such as chitosan, carboxymethyl cellulose and a gelatin-glycerin solution. In addition, we did a comprehensive analysis of its durability according to the resistances to acid, alkali, oxidation and dry heat aging, and also further investigated the effects of conservation on handwriting. The results show that 1) the HDI trimer performs better than other reinforcement materials in terms of safety; 2) with regard to durability, the HDI trimer has good conservation effects on handwriting materials of Chinese ink and carbon ink, but relatively poorer conservation effects on those of blue-black ink, pure blue ink, red ink and pencil. In addition, in order to achieve ideal reinforcement effects, appropriate mass fractions of the reinforcement material should be chosen according to the characteristics of different writing materials.