Molybdenum disulfide (MoS2) has been widely used in vacuum environment as an excellent solid lubricant. However, the application of MoS2 is greatly limited in terrestrial atmosphere due to the sensitivity to humidity. Although the sensitivity of MoS2 to water vapor has been widely recognized, the mechanism is not clear. To explore the tribological mechanism of MoS2 in the presence of water vapor, a series of experiments were performed to investigate the effect of N2 (inert gas), O2 (active gas), air (a combination of both) and cyclic humidity change in air on the frictional response of MoS2 to humidity. According to the results, a model that described water adsorption enhanced by active sites in MoS2 and formed oxides, and an adsorption action change in water molecules with humidity was proposed. The model was applied to explain the recovery and instantaneous response of friction coefficient to humidity change.