Traditionally TSR-solid bitumens (pyrobitumen) have been recognized as direct, stable products of the thermochemical process. Compared with hydrogen sulfide (H2S), the presence of these solid bitumens has not been considered an important factor in TSR In this paper, activated carbon (C) was selected as a model compound for solid bitumen (pyrobitumen), and thermochemical reduction of calcium sulfate (CaSO4) by activated carbon (C) was carried out under hydrothermal conditions at elevated temperatures. Thermodynamic characteristics of the system of CaSO4-C-H2O were investigated. According to the experimental results, CaCO3, H2S and CO2 were determined as the main TSR products. The threshold temperature for initiating TSR in the present study was only 300 ℃, which was much lower than those of most previous TSR simulations using hydrocarbons and in accordance with the further thermodynamic calculations. Process simulation of TSR was conducted using the software of HSC Chemistry 5.0. It was found that at reservoir temperatures of 25℃-200℃, TSR in the system of CaSO4-C-H2O was totally controlled by kinetic factors. Under a constant temperature, the increasing pressure was unfavorable to the initiation of TSR. The intensity of TSR was closely associated with the amount of water. The small amount of water may contribute to better oxidizing conditions. The influence of water content on TSR may be related to the saturation concentration of CaSO4. When pH≤2, sulfate reduction rates are dependent upon the decrease of pH at a certain temperature. However, in the pH range of sedimentary basin formation water (pH>4), effect of pH on TSR is negligible. TSR in the system of CaSO4-C-H2O was an exothermic process. The released heat increased with the increasing temperatures, which was estimated as about12.9 J-133 J/(mol)CaSO4 at 25 ℃-200 ℃. Thermodynamic studies and the experimental results implied that solid bitumens (pyrobitumen) more easily involved in TSR than gaseous or liquid hydrocarbons.