The pyrolysis of oil shale is a complex process including a myriad of chemical reactions. A widely approved understanding suggests a two-step decomposition process for oil shale pyrolysis, considering bitumen as the intermediate product. In this study, intermediates derived from various pyrolysis conditions are comprehensively studied by FTIR, GC, GC-MS and NMR methods to understand the pyrolysis mechanism of oil shale and composition feature of intermediate. The pyrolysis of oil shale is a dynamic process, and the results show that the intermediate is continuously generated before 400 A degrees C, accompanying with the formation of final products from both intermediate and kerogen. The maximum yield of intermediates is presented at the fastest oil-producing temperature range (375 A degrees C in this study). Carbon chains in intermediate become short with the increase in temperature. Most components in pyrolysis intermediate are long straight aliphatic chains; thus, intermediate is much heavier than shale oil. Further reactions make intermediate convert into shale oil product. Aliphatic hydrocarbons occupied the biggest proportion over 86% at 375 A degrees C, mainly in the form of straight-chain alkanes. A few parts of aromatic fragments with small ring numbers will also transfer into intermediate. The heteroatom-containing compounds are mainly alcohols, ketones, amides and halohydrocarbons. High aromaticity in shale oil at high temperatures can be attributed to the condensation reaction of abundant aliphatic hydrocarbons in intermediate. During the conversion process from intermediate to final products, the generating capacity of oil is evidently higher than that of gas.