In order to study the evolution of physico-chemical structure during hydrothermal upgrading process and its influence on moisture re-adsorption behavior, a lignite from Inner Mongolia was selected and hydrothermally upgraded at 170330 ℃ and the moisture re-adsorption behavior of the upgraded lignite was investigated.Moreover, the physical and chemical structures of the lignite were characterized by chemical titration and nitrogen adsorption.The results show that great changes occur on both physical and chemical structures during hydrothermal upgrading process.Pore volume and specific surface area increase and then decrease with temperature increasing.Carboxyl and phenolic hydroxyl groups would decompose after 200 ℃ and 230 ℃ respectively.The combined effect of oxygen-containing functional groups and pore structure changes the equilibrium moisture content (EMC) of upgraded lignite.In order to reduce the moisture re-adsorption ability of lignite, the upgrading temperature around 300℃ should be selected to destroy pore structure and oxygen-containing functional groups while ensuring the subsequent transformation of upgraded lignite.