Abstract: Methane adsorption in shale is the result of gas-liquid-solid interaction when considering water saturation in actual condition. And the moisture (water saturation) which significantly influences methane adsorption capacity will likely make shale gas resources misestimated. In this paper, we analyze the interaction characteristics between methane, water film and clay base on adsorption theory, and results reveal that:(1) methane adsorption on clay (dry) could be described by gas-solid interface Langmuir adsorption equation; (2) methane adsorption on water film could be described by gas-liquid interface Gibbs adsorption equation; (3) gas-liquid-solid interaction could be described by ‘gas-solid’ and ‘gas-liquid’ integrated equation. Meanwhile, we find that water saturation distribution is significantly effected by pore size, and micropores could be filled with water in certain condition while macropore only bound by water film. Therefore, the influence of moisture on methane adsorption is mainly for two aspects:(1) micropores which blocked by water are invalid for methane adsorption; (2) macropores bounded by water film change the interaction characteristics for methane adsorption (from gas-solid interaction to the gas-liquid interaction), and the overall effect could decrease the adsorption capacity by 90% in our study. Our present work reveals mechanism of moisture effect on the shale absorption capacity and lays the foundations of evaluating the adsorbed gas in shale gas reservoir more accurately.