During the thermal enhanced oil recovery (EOR) process, the hazardous hydrogen sulfide (H₂S) gas among the produced gases causes significant difficulty in the exploration and development of petroleum. In this study, the effects of superheat degree on the H₂S generation by heavy oil aquathermolysis were explored through simulated experiments. The crude and residual oils before and after the reaction were separated into saturate, aromatic, resin and asphaltene fractions (SARA). The oil samples were analyzed from various perspectives by various characterization methods including Fourier transform infrared (FTIR) spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy (XPS). The results showed that H₂S generation was favored by larger superheat degree at the same temperature, and it increased from 0.178 to 0.345 mL g⁻¹ oil with an increase in the superheat degree from 62.19 to 89.42 °C. The contents of the sulfur-containing substances, which were supposed to be the main sources of H₂S generation, in the saturate and aromatic fractions decreased significantly with an increase in the superheat degree; the increase in the superheat degree led to a slight reduction in the contents of the methylene, methyl and carboxyl/carbonyl groups. Moreover, the analysis of the main sulfur existing forms before and after the reaction suggests that sulfur in the forms of sulfides, sulfones and sulfates is more likely to generate H₂S under superheated steam conditions. This study provides an understanding of the mechanism of H₂S generation during the process of injecting superheated steam for heavy oil recovery.