The sealing clay is the key physical example of the document sealing system of the Qin dynasty in ancient China. However, only the inscriptions and aesthetic values of the clay were discussed untill now, and the relevant sources were not traced from the perspective of scientific analysis. A total of 81 sealing clays unearthed in Xi’an were studied by means of ultra-depth of field microscopic observation, petrographic microstructure analysis and X-ray fluorescence spectroscopy. The relevant methods of tracing and making of mud samples are discussed based on the results of literature investigation and element analysis. The composition, technology and spatial links between different sealing clay collected from all over the country show that the highly organized sealing materials and systematic processes are an important part of the establishment of unified China, in order to provide detailed and effective scientific information for the subsequent sealing clay protection and archaeological research.