Abstract: The macromolecular coal structural model is constructed and the H2 O adsorption behavior in coal is simula- ted using Grand Canonical Monte Carlo method at 1-100 kPa and at 298. 15,303. 15,313. 15 and 318. 15 K to inves- tigate the H2 O adsorption mechanism on molecular scale which lays a theoretical foundation for further research invol- ving the interaction between coal and water. The results show that the adsorbed amount and isosteric heat of H2 O show a negative correlation with temperature and a positive correlation with pressure. When pressure is greater than a critical pressure,there is a plateau area of potential energy probability distribution between two adsorption sites indicating equal proportion. When exceeding the pressure,the adsorbed amount increases sharply then capillary condensation oc- curs with water cluster because of the control of stronger hydrogen-bond interaction. With the increase of pressure,the adsorption process transfers from weaker adsorption sites provided by H2 O-coal interaction to stronger adsorption sites provided by H2 O-H2 O interaction,until total sites are secondary adsorption sites forming from adsorbed H2 O then the isosteric heat tend to be steady. The 3D probability density distribution provides a more intuitionistic comprehensive understanding to H2 O adsorption behavior and mechanism in coal.