Methane adsorption in shale is a gas-liquid-solid interaction rather than a gas-solid interaction by considering the initial water saturation in actual condition. As an important constituent of inorganic matter, clay minerals may affect gas-in-place of shale systems. Generally, Clay minerals are strongly hydrophilic with a water film bound on its surface, significantly reducing gas sorption capacity, which will lead to overestimate gas-in-place (GIP) of shale gas reservoir. In this work, we analyze the interactions between methane, water film and clay, and results reveal that: methane adsorption on clay (dry) is a typical gas-solid interaction; however, methane adsorption on clay bound water film should belong to gas-liquid interaction. Furthermore, a unified model is established to describe gas-water-clay interactions, in which, gas-solid Langmuir equation and gas-liquid Gibbs equation are integrated by water coverage coefficient. Meanwhile, a mathematical model is presented to quantify thickness of water films by considering surface force interactions between liquid film and clay. Our results show that, the water film thickness in shale clay pores mainly depends on relative humidity and pore size. Under a certain humidity condition (such as 0.98), the water saturation distribute in different sized pores mainly as: (i) capillary water in the small pores (<6 nm); (ii) water film in the larger pores. Thus, considering the water distribution characteristics, the effect of moisture on methane adsorption capacity is mainly for two aspects: (i) small pores (<6 nm) blocked by water are invalid for methane adsorption, (ii) large pores bounded by water film change interaction characteristics for methane adsorption (from gas-solid interaction to the gas liquid interaction). The overall effect could reduce the adsorption capacity by 80%-90%. (C) 2016 Elsevier B.V. All rights reserved.