In this article, thermal simulation experiments on the reactions in TSR process (CH4-CaSO4, C2H6-CaSO4, and C3H8-CaSO4) were carried out using autoclave at high temperature and high pressure. The gaseous and solid products were characterized by some advanced analytical methods. On the basis of the experimental results, the reaction mechanisms were tentatively investigated. It was found that three reactions can proceed to produce H2S, H2O, and CaCO3 as the main products at the temperature range of 450-700 degrees C. The high simulation temperatures also resulted in the production of undesired material. The results obtained in this article can provide important information for the investigation on the natural gas destruction in gas reservoirs.