Mo-based catalysts are widely used for the SO2 hydrogenation process. However, the detailed reaction mechanism is still unclear and some details should be further supplemented. In this paper, the SO2 hydrogenation processes over the Mo-based catalyst were systematically studied. Several technologies including temperature-programmed experiments, isotope-tracing experiment, FTIR spectra and switching experiment were adopted to investigate the reaction steps. The results indicated that during the SO2 hydrogena