The generation of H2S causes many problems during steam injection thermal recovery. To make clear the formation rule of H2S under the condition of steam injection thermal recovery, tetrahydrothiophene was selected as a model compound, and then reaction mechanism and kinetics were explored based on indoor thermal simulation. The results showed that tetrahydrothiophene aquathermolysis reaction can generate a large amount of H2S. The C-S bond formed by heteroatom S broke at first and then opening-ring occurred, which produced small molecular alkanes or alkenes. The decarbonylation of aldehydes and other organic compounds resulted in the formation of CO, hydrocarbons, and other related compounds. High-temperature water played a key role in the process of tetrahydrothiophene aquathermolysis reaction and would directly participate in the elementary reactions such as hydrolysis and water-gas shift reaction (WGSR). These elementary reactions would finally lead to the migration of hydrogen from water to gas-phase or liquid-phase products. In addition, a kinetic model of H2S formation during tetrahydrothiophene aquathermolysis reaction was established through kinetic experiments, the activation energy is 54.55 kJ/mol, and the frequency factor is 166.47 day(-1).