Rare-earth oxides of La2O3 and CeO2 were used as promoters in the preparation of K2MoS4/SiO2 catalyst for methanthiol synthesis from syngas blending with H2S. The experiment results showed that the addition of La2O3 or CeO2 to K2MoS4/SiO2 was able to improve the catalytic performance in methanthiol synthesis both in activity and selectivity. La2O3 displayed more beneficial effects than CeO2 and the catalyst modified with La2O3 presented a selectivity of 98.6% for methanthiol. In the catalysts modified with La2O3 or CeO2, less K2MoS4 decomposed to MoS2 and more to the Mo-S-K phase, which is favorable and active for methanthiol synthesis, was determined by XRD. XPS results revealed that La2O3 was able to protect molybdenum species from deep reduction and to raise the concentration ratio of Mo6+ to Mo4+ in the catalysts, and also to depress the formation of (S-S)(2-) species which may induce the formation of hydrocarbons.