In this study, a series of three-component CoSx/gamma-Al2O3 catalysts were developed for the reduction of high-content SO2 to sulfur in smelter off-gas. The introduction of a small amount of lanthanum additive greatly improved the catalytic activity and stability of Co-Cu/gamma-Al2O3 catalysts using coal gas as reductant. The catalysts were characterized by XRD, XPS, BET, and SEM techniques. H-2/CO-TPR was recorded to study redox properties of catalysts. With the use of a CO-H-2 mixture with different proportions, a series of parallel experiments were performed to investigate the mutual impact and relation of CO and H-2. Under optimum conditions, SO2 conversion of 99% and sulfur selectivity of 99% were obtained. The catalyst remained highly active after a long run of 200 h. Process simulation was used to investigate the molecular behavior of catalyst. Combined with in situ infrared characterization, the catalytic mechanism was put forward and verified.