Abstract: During the thermal recovery of extra heavy oil in the Liaohe Oilfield, the concentration of H₂S increased, which results in the increased cost of the desulfurization facility and oil-and-gas treatment. Crude oil, associated gas, formation water and reservoir minerals were analyzed and the H₂S production had no significant correlation with the sulfur content in the crude oil and the SO₄^2- concentration in the formation water, but had a good covariance with the pyrite content in reservoir. The sulfur in pyrite had biological source, and its isotopic range was basically consistent with that of crude oil, which originates in the thickening stage of crude oil and forms in large quantities in the thermal recovery stage of heavy oil. Except for the thermal decomposition of organic sulfur compounds (TDS) and thermochemical sulfate reduction (TSR), the thermal simulation experiment shows that the decomposition of pyrite is also one of the ways to produce H₂S in the process of steam thermal recovery. When injected, lower salinity steam dilutes the formation water and pyrite decomposition becomes the main source of H₂S. The production concentration of hydrogen sulfide was controlled by reservoir geology, the thermal recovery method, heating time and temperature.