To address the complex changes in pyrolysis reactions and the difficulty of temperature control in in-situ thermal recovery of oil shale deposits，thermogravimetric analysis experiments were conducted to segment the pyrolysis process of the Chang 7 oil shale in Ordos Basin. X-ray diffraction（XRD）and gas chromatography-mass spectrometry（GC-MS）were utilized to examine the composition of products during the organic matter pyrolysis and exothermic stage. The main chemical reactions evolving at different stages were analyzed to further validate the segmentation. The results showed that the pyrolysis of oil shale under a nitrogen atmosphere could be divided into three stages：dehydration（＜300 ℃），organic matter pyrolysis and exothermic stage（300—720 ℃），and mineral endothermic decomposition（＞720 ℃）. During the organic matter pyrolysis and exothermic stage，the mass loss of oil shale accounted for approximately 74% of the total mass loss，with no thermal decomposition of inorganic minerals. This stage could be further subdivided into three phases：low-temperature initial phase （300 ℃≤T＜450 ℃），mid-temperature pyrolysis phase（450 ℃≤T＜600 ℃），and high-temperature pyrolysis completion phase（600 ℃≤T≤720 ℃）. The primary pyrolysis of organic matter occurred in the mid-temperature phase. As the temperature increased，dehydrogenation reactions of alkanes and esterification reactions transitioned to the thermal cracking of esters and the chain scission of alkanes producing small molecular hydrocarbons. When the temperature rose to 600—720 ℃，compounds tended to polymerize into semi-coke. At the mid-temperature pyrolysis phase， the composition of shale oil was optimal， including 27.70% esters， 29.89% aliphatic hydrocarbons，14.49% alcohols and 12.66% aromatic hydrocarbons. These findings provided theoretical guidance for analyzing the pyrolysis mechanism of oil shale，establishing numerical simulation models for oil shale，and improving on-site recovery rates. They also offered a basis for controlling the reaction temperature range of organic matter to ensure a high conversion rate.