Ancient milk residues research is a hot spot of international scientific archeology. Major analytical methods currently in use include stable carbon isotope composition of individual fatty acid and protein immunoassay. Such research has not been developed in China. In this research, a commercial ELISA kit was adopted to analyze the black block residue from the Subeixi Site in Shanshan county, Turpan district, Xinjiang Uygur Autonomous Region. ELISA analysis demonstrated the presence of a small amount of bovine casein in the residue. Thus, the detected residue could be a milk product or a certain substance blended with milk. It suggested that the milk had been included in an ancient diet no later than the third to fifth century B.C. among the people of the Xinjiang area. This analytical method is well developed, and could be widely applied to the fast detection of ancient milk residues in China.