The by-product SO3, generated by the conventional catalyst V2O5-WO3/TiO2 employed in selective catalytic reduction (SCR) systems, has garnered attention in pollution control efforts. Studies on existing control have predominantly concentrated on the tail gas section, with insufficient understanding of in-situ suppression theories and methods at the source of SO3 generation. We propose for the first time the introduction of low-valence reduced sulfur to synthesize a novel SCR catalyst, V2O5-WS2/TiO2, which elucidates the remarkable effect and in-situ mechanism of action in inhibiting SO3 formation at its source. Compared to conventional catalysts, the new catalysts showed enhanced SO3 inhibition, particularly at 350 °C, where SO3 production was 64.1 % and 33.9 % lower than that with VTi and VW/Ti catalysts, respectively. This was due to their reduced surface area, smaller pore volume, and fewer alkaline sites, which hindered SO2 adsorption and oxidation. The results revealed that S2− on the catalyst surface reacts with V5+, reducing it to V4+ and V3+. This reaction hinders the interaction between V5+-OH and adsorbed SO2, thereby reducing the formation of VOSO4 intermediates. Furthermore, the WS bond in WS2 impedes the formation of intermediate HSO4−, and the reduction of these intermediates ultimately leads to a decrease in SO3 generation. Additionally, the incompletely depleted S2− reacts with the final SO3 produced, forming SO2 and SO42−, which further decreases the SO3 generation rate and supports the maintenance of a reduced state on the catalyst surface. It is evident that low-valence reduced sulfur plays a crucial role in inhibiting SO3 generation.