In the unearthed relics, most of the organic residues are attached to the objects or the surface of archaeological sites. Organic residues are often neglected in previous studies, because they have no definite shape, sometimes the amount of them is small and difficult to be found and preserved. However, as materials for ancient human life, this kind of organic residues contain important historical information and have cherished value. Even through the organic residues found in objects or sites, it can determine the use of objects and even sites. Ancient illuminating fuel belongs to such kind of organic residues. Animal tallow, vegetable oil and wax were used in ancient China. Vegetable oil and wax have been studied in the past, but the research of animal tallow only stays in distinguishing ruminants from non-ruminants. In this paper, pyrolysis gas chromatography-mass spectrometry (Py-GC/MS) was introduced to study organic residues. Triglyceride and production in aging process could be completely decomposed into glycerol and fatty acid due to the loss of acyl group by pyrolysis temperature of 600 ℃ and pyrolysis time of 12 s. By calculating the relative content of fatty acid, the characteristics of different animal oils could be analyzed. At the same time, high temperature pyrolysis replaces the traditional pretreatment method of acidizing and purifying samples, which can retain the components of samples to the greatest extent. With the addition of excessive tetramethylammonium hydroxide, the fatty acids and alcohols in the sample can be converted into corresponding esters and ethers. The methylated sample can be separated at the initial 50 ℃ of gas chromatography for 5 minutes, then elevated to 280 ℃ for 10 minutes. The methylated sample components can be determined by mass spectrometry at a split ratio of 1∶100. The residual illuminating fuel from an iron lamp, which was excavated in a tomb from Yihenaoer, Inner Mongolia, were analyzed. According to the results, the main components of residues were fatty acids with continuous number of carbon atoms ranging from 7 to 22, in which relative content of tetradecanoic acid, pentadecanoic acid, hexadecenoic acid, heptadecanoic acid and octadecanoic acid were higher. Compared with the compounds of lard, beef tallow, and mutton tallow (three main kinds of animal tallows used as illuminating fuels), the contents of odd number of carbon atoms of fatty acids in mutton tallow were significantly more than the other two kinds of animal tallous. And this feature was also verified in archaeological sample. By combination with the other researchers’ results, the exact type of animal tallow contained in residues should be mutton tallow. Moreover, the hydrocarbons and alcohols contained in the archaeological sample were identified as beeswax according to the relative contents of fatty acids and alcohols. Thermally assisted hydrolysis-methylation pyrolysis-gas chromatography-mass spectrometry (THM-Py-GC/MS) provides a new method for the analysis of organic residues, especially animal tallous. In addition, a useful attempt has been made to identify the species of mixed organic residues.