Abstract: To grasp the influence of changes in the content of key functional groups on the coal spontaneous combus⁃ tion during the secondary oxidation process，the samples of gas fat coal and long flame coal were collected from Ewirgol and Liuhuanggou in Xinjiang，China. The two coal samples were treated at different pre⁃oxidation temperatures(100， 200，300，400 and 500 °C)，and the oxidation time was 40 min. The secondary oxidized coal samples were conducted by the experimental tests from the thermogravimetric and in⁃situ diffuse reflectance infrared spectroscopy. The raw coal used as a control group. Thermal gravimetric experiment was used to analyze the mass change characteristics of the coal during the thermal mass loss process(30-800 °C). Based on the three characteristic temperature points(T2， T5，T7) where the rate of mass change was 0，the coal sample oxidation process was divided into three stages. The changes of key functional groups during the heating process coal samples were analyzed by in⁃situ infrared spectrometer. The results show that the main reaction temperature range of hydroxyl is Tc-T1(Tc is room temperature，and T1 is critical temperature)，and there is almost no change in the late stage of the reaction in the coal⁃oxygen reaction. Among the different pre⁃oxidized coal samples of two coal samples with the largest increase in hydroxyl content is L500 and A400(L500 is the Liuhuanggou coal sample with the pre⁃oxidation at 500 °C，and A400 is the Ewirgol coal sample with the pre⁃oxidation at 400 °C). The main reaction stage of aliphatic hydrocarbons，aromatic hydrocarbons and oxy⁃ gen⁃containing functional groups is T2-Tz(T2 is the cracking temperature，and Tz is final temperature). When the tem⁃ perature is lower than T2 ，these functional groups are less involved in the reaction. The content of key functional groups of coal samples changes significantly after being treated at different pre⁃oxidation temperatures. Excessive high pre⁃oxi⁃ dation temperature will reduce the content of aliphatic hydrocarbons，aromatic hydrocarbons and oxygen⁃containing functional groups in coal. When the pre⁃oxidation temperature is 200 °C，comparing the changes in functional groups of coal samples with different pre⁃oxidation temperatures，the aromatic hydrocarbons，oxygen⁃containing functional groups and aliphatic hydrocarbons which from coal molecules all reach their peaks.