Abstract: An Mengdong lignite was modified by hydrothermal method. The water binding energy between the coal surface skeleton molecules and various oxygen containing functional groups was determined and analyzed by means of micro calorimeter. The results show that carboxyl and carbonyl significantly reduced with the increase of temperature, while the change of hydroxyl is complex. The order of hydrophilic ability from large to small for oxygen containing functional groups on coal surface is carboxyl, phenolic hydroxyl, carbonyl, large molecular of the coal.