Abstract: It will produce acid gas such as H₂S and CO₂ in heavy oil recovery by steam injection because of the complicated reaction which induced by thermochemical sulfate reduction (TSR) between stream and heavy oil in reservoir. In this paper, model compound of heavy oil (n-hexadecane) and four metal salts (MgSO₄, Al₂(SO₄)₃, Na₂SO₄ and CaSO₄ were taken as research objects, and the H₂S formation mechanism of TSR experiments was studied. The results show that the products are carbonyl sulfide, mercaptan, thiophene, thioether and gases (H₂, CO₂, H₂S, MgO, hydrocarbons). The order of H₂S formation in different reaction systems is Al₂(SO₄)₃ > CaSO₄ > MgSO₄ > Na₂SO₄, but the order of CO₂ is Al₂(SO₄)₃ > Na₂SO₄ > MgSO₄ > CaSO₄. By comparing the generation order of H₂S and CO₂ in different systems, it is known that the greater the number of metal cation charges is, the stronger the autocatalytic effect is, and the reaction paths of different sulfate systems are different. The TSR way of n-hexadecane with magnesium sulfate was discussed, including protonation, hydrocarbon elimination reaction, thiosulfate conversion to organic sulfide, hydrogen sulfide autocatalysis, pyrolysis and hydrolysis of sulfide, in which hydrogen sulfide autocatalysis is the main process. Finally, the reaction kinetics was studied and the calculated activation energy is 61.498 kJ/mol.