Abstract: In order to prevent potential safety hazard which is caused by hydrogen sulfide(H₂S) produced from wellhead during steam injection in heavy oil reservoir, and increasing safe production level of oil well, it is necessary to carry out experimental research on the source and formation mechanism of H₂S during thermal recovery of heavy oil. This study conducted sulfur measurement, isotope analysis, and thermal simulation experiment of H₂S generation using core, crude oil, and output water samples from the Wa-38 block in Xiaowa oilfield, a heavy oil block in Liaohe oilfield, Panjin, China. During thermal recovery of heavy oil, H₂S is mainly derived from core and heavy oil. In the sulfur isotope fractionation process, the δ34S value of sulfide product (H₂S) reflects the fractional characteristics of sulfur at high temperature during thermal chemical reduction of sulfate. Regarding the formation mechanism, H₂S is mainly formed by interaction between aquathermolysis of heavy oil and thermal chemical reduction of sulfate under high-temperature, high-pressure and acidic conditions.