MoS2-based catalysts showed great promise for converting CO/H-2/H2S gas mixture into methanethiol (CH3SH). However, the active phases for this reaction were unexplored widely. In the present study, we tackle this issue through investigating the effect of potassium (K) and supports on the nature of MoS2 catalysts via the characterizations of XRD, UV-vis, TPS, XPS, HR-TEM, as well as HAADF-STEM combined with energy-dispersive spectroscopy (EDS) etc. The results suggested that K species can not only lead to formation of different oxidized precursors but also affect the microstructure, surface morphology and electronic properties of active MoS2 phases. Three sulfided species, i.e., pure MoS2, K-decorated MoS2 and K-intercalated MoS2 nanoslabs, were simultaneously detected, and were demonstrated to be derived from the reduction and sulfidation of oxidized precursors via O-S exchange mechanism. Furthermore, the activity results showed that pure MoS2 species worked only for the reaction of CO with H2S to COS, while K-decorated MoS2 and K-intercalated MoS2 nanoslabs exhibited unique ability for the hydrogenation of COS to produce CH3SH. K-decorated MoS2 nanoslabs formed mainly on mesoporous SBA-15 support, which possessed higher CH3SH selectivity than that over K-intercalated MoS2 generated primarily on SiO2 support. In addition, the further improvement of CH3SH selectivity depended on the properties of support with respect to the dispersion, the high dispersion facilitated the formation of the increased amount of Mo-coordinatively unsaturated sites (CUS).