i>Siku Quanshu is the largest cultural project in ancient China and also belongs to the precious cultural heritage of all mankind, having important literary and cultural relic value. The paper used for its writing was clearly recorded in the Qing Palace archives, but so far no one has analyzed the real paper samples using scientific methods. In this study, we conducted a comprehensive analysis on a paper sample from the Wenjin Pavilion’s Siku Quanshu version of Nanxun Shengdian from the “Okura Collection” of Peking University Library, using various scientific methods. These included optical microscopy (OM), fiber dyeing analysis, scanning electron microscopy and energy dispersive spectrometry (SEM-EDS), micro computed tomography (micro-CT), micro X-ray diffraction (micro-XRD), and attenuated total reflection Fourier transform infrared spectrometry (ATR-FTIR). The aim of this study is to explore the morphology, fiber composition, chemical composition, and structure of this Siku Quanshu paper sample in order to confirm the type of paper and explore its processing techniques, which could reveal the scientific value of the paper and lay a foundation for the better preservation of these books. In addition, the famous “Jinxian Bangzhi” in Chinese papermaking history was also explored. The results of OM and SEM show that both sides of the paper were the same in microstructure, and there were white granular filler materials between the fibers. The result of micro-CT show that the paper had a single layer and the filler material was irregularly distributed inside the paper rather than just aggregated on its surface. Through the fiber analysis, it could be confirmed that the fiber composition of the paper was wingceltis bark and rice straw fibers, thus proving that the paper sample belonged to Xuan paper. According to the SEM-EDS result, the main elemental components of the paper were C, O, Si, S and Ca, and the distribution of S and Ca had a high consistency with the filler area, indicating that S and Ca might be the main elements of the filler. By using micro-XRD, the main components of the paper were confirmed to be cellulose and gypsum, with a small amount of calcium oxalate crystals and quartz crystals. Cellulose is the main component of plant fibers, calcium oxalate crystals are an ergastic substance of wingceltis bark fibers, and quartz should come from the epidermal cells of rice straws, so the main component of the filler material should be gypsum. The ATR-FTIR analysis result also confirmed that the compositions of both sides of the paper sample were all the same—including cellulose, gypsum, and a small amount of calcium oxalate. The micro-XRD and ATR-FTIR results were consistent with each other. Therefore, it could be determined that gypsum was the main component of the filler material between fibers. The papermaking processes of this sample are also discussed in the article. Based on the distribution status of gypsum and the technical history of papermaking, it could be speculated that the gypsum was more likely to be added to the pulp. However, it is also possible that the gypsum was converted from calcium carbonate in the paper. Based on the above results, it is concluded that the paper used for Wenjin Pavilion’s Siku Quanshu version of Nanxun Shengdian from the “Okura Collection” is single-layer Xuan paper made from wingceltis bark and rice straw fibers with gypsum randomly distributed on the surface and inside of the paper. It is speculated that the gypsum may have been added to the pulp or converted from calcium carbonate in the paper. The results also confirm that the “Jinxian Bangzhi” mentioned in the literature is Xuan paper based on conclusive evidence. Given that the fiber materials of this paper sample are different from those made from pure wingceltis bark fibers in previous studies, it is speculated that the “Jinxian Bangzhi” should be a general term for certain specific types of Xuan paper, which involves different raw materials and processes. Due to the long completion time of Siku Quanshu, there may be some differences in different batches of paper used for writing. If we want to know whether all the paper used for the Siku Quanshu stored in the Northern Four Pavilions belongs to this kind of Xuan paper, it is necessary to systematically analyze and compare more paper samples.