The cooperative transition of sulfur-containing pollutants of COS/H₂S/H₂ to the chemical feedstock of methyl mercaptan (CH₃SH) catalyzed by Mo-based catalysts has good application prospects. However, the role of rare earth elements as promoters in this reaction system has not been investigated currently. In this study, the effects of the rare earth of lanthanum (La) on the catalytic performance of Mo-based catalysts were investigated by XRD, Raman, TEM, H₂-TPR, and XPS characterizations. The results showed that the addition of La species led to the formation of larger particle size and lower sulfidation degree of Mo species, but the higher, rather than lower, catalytic activity was obtained. This unusual phenomenon is confirmed to be due to the generation of La₂(MoO₄)₃ crystals covered with surface MoS₂ layers caused by the interaction of Mo oxide and La species at the sulfidation atmosphere. The presence of La₂(MoO₄)₃ changes the electronic and chemical environment of MoS₂ in the catalyst, which lead a positive effect on the catalytic activity, outweighing the negative effect caused by the larger particle size of Mo species and the reduced sulfidation.