To study the properties of starch paste with alum and its influence on paper-based mounting pieces, the amount of alum added in traditional paste preparation recorded in relevant documents was examined. The effects of alum addition on paste properties in different procedures of painting and calligraphy relic mounting were studied, as well as its effects on the pH value, color difference, stiffness, tensile strength, tearing degree, and folding resistance of paper mounting pieces. The results show that starch paste with m(starch)∶m(alum)=100∶2 had a higher disintegration value, a lower retrogradation value, and a slightly lower final viscosity than the paste without alum. In practical applications of mechanical paste to painting and calligraphy works, the addition of alum significantly reduced the paste’s pH value, although the accumulated pH value fluctuated slightly with storage time. The viscosity of the alum-added paste was slightly lower than that of the paste without alum, and while the viscosity of the thick paste was better, its adhesive strength became slightly lower in a cold storage environment. Additionally, the alum-added paste had some antibacterial properties. In terms of the properties of mounting piece samples made with the two types of paste, their pH values and color differences showed no obvious impact. After aging, the stiffness of the sample made with alum-added paste was significantly higher than that of the sample made with alum-free paste. The former also performed slightly better in tearing degree and folding resistance, while the tensile strengths of the two groups of samples were basically the same. This study provides theoretical guidance for the use of alum in paste used for mounting painting and calligraphy works, contributing to the scientific development of traditional techniques.