The Ziku Street site in Qingyang District, Chengdu, has been identified as a waste deposit associated with “Shu Xigong” in the Qin Dynasty. A lacquer ash waste sample was recovered during the sorting of materials from this site. Through scientific analyses, investigations of modern craftsmanship, and simulation experiments, this study examined the lacquer ash waste to elucidate the lacquering techniques in Qin Dynasty “Shu Xigong”. The analytical results show that the lacquer ash waste consisted of two distinct layers:a light-brown lacquer film on the surface, and an inner layer containing a significant amount of clay particles. Its organic components mainly included Chinese lacquer and drying oil. The comparison with lacquering techniques of Chengdu Lacquerware Craft Factory indicated clear compositional differences, particularly in the presence of added oil. The simulation experiments further demonstrated that the addition of drying oil reduced the hardness and adhesion of lacquer ash, while improving its flexibility, thereby better accommodating deformation requirements during use, and enhancing overall performance. In addition, drying oil prolonged the curing time of lacquer paste, which might also relate to the timing of its application. As an official handicraft workshop of the Qin Dynasty, “Shu Xigong” likely represented the most advanced technical level of its time. This study reveals the craftsmanship and performance characteristics of lacquer ash, contributing to the reconstruction of lacquer ash production processes used in weapon production at “Shu Xigong”, and providing insights into the lacquering techniques of Qin Dynasty official artisans.