When a fracturing fluid invades a nanoscale shale pore structure, it will inevitably affect the ad-/desorption behavior of methane adsorbed on a shale pore wall, impacting the overall methane production of a well. In this work, methane ad-/desorption experiments were conducted on the Longmaxi formation shale with varying levels of relative humidity (0, 0.1, 0.2, 0.3). In addition, the mechanisms and efficiency of methane replacement by water molecules were analyzed. The results indicate that water molecules occupy the adsorption sites of methane molecules in shale nanopores, thus reducing the methane adsorption volume and increasing the difficulty of adsorption. With an increase in the water content, the Langmuir volume of methane adsorption decreases linearly. At high pressures (greater than 8 MPa), a higher water content results in a lower hysteresis index, while at pressures less than 8 MPa, the hysteresis index increases and results in increasing methane desorption compared to dry shale samples. The efficiency of methane replacement by water molecules increases with an increase in the water content and decreases with an increase in pressure. The results of calculating the disjoining pressure of a shale surface-methane adsorption layer-water film system show that the stability of a water film on a shale surface is far greater than that of a methane adsorption layer on a shale surface. These insights will be helpful for analyzing the influence of an imbibition fracturing fluid on shale gas production under in situ conditions.