The desulfurization mechanisms of thiophene and 2-methyl thiophene were investigated by the density functional theory (DFT) during pyrolysis under water vapor atmosphere. All possible reaction pathways of these desulfurization mechanisms were explored at M06-2X/6-311g (d) level. The Multwfn3.0 and VMD1.9.2 programs were used to analyze weak interactions between thiophene compounds and H2O molecule. It can be seen that hydrogen bonds can be formed in the reactions of thiophene sulfurs and H2O. Since H2O molecule can decompose at higher temperature and generate free radicals, such as·H and·OH,, the desulfurization mechanisms of thiophene and 2-methyl thiophene with free radicals need to be further considered. The reaction energy barriers (∆G≠) and reaction energies (∆GP) of thiophene and 2-methyl thiophene with H2O molecule (g) or free radicals (·H and·OH) have been stimulated and calculated in detail. Based on the transition state theory (TST), the rate constants corresponding to these elementary reactions are also calculated, meanwhile the speed and spontaneity of every reaction can be obtained from the aspect of kinetics. Theoretically, it is found that H2O (g) directly attacking C-S bonds of thiophene and 2-methyl thiophene cannot easily generate COS and H2S even at 1200 K in terms of thermodynamics and kinetics. If the desulfurization mechanisms of thiophenes are investigated by free radicals mechanisms under steam atmosphere, their initial energy barriers needing to be overcome significantly reduce. Therefore, desulfurization mechanisms of thiophenes and H2O (g) are the most possibly dominated by radical reactions at higher temperatures and H2S is mainly generated.