Abstract: In this study,the moisture in a lignite was removed by low-temperature drying method,and the state of mois- ture in lignite,the chemical composition of lignite,the characteristics of surface pore structure and micro-morphology during dehydration were studied by low field nuclear magnetic resonance, Fourier-transform infrared spectroscopy, physical adsorption and scanning electron microscopy. Combined with the change of conversion rate of lignite in coal /oil hydrogenation and co-refining process before and after dehydration,the effects of moisture in lignite on the physical and chemical structure of lignite and hydro-conversion performance were further understood. The results show that the removal of moisture in lignite is divided into two stages. The first stage is the quasi-steady-state drying stage,the water is basically removed,and the removal speed is faster. The second stage is the slow-down drying stage,the amount of re- moval is small. The water in the lignite has four states:non-freezing water,adsorbed water,inter-particle water and free water. The adsorbed water and the non-freezing water account for a large proportion,and are easily removed during the quasi-steady-state drying stage,and the remaining water more presents in the pores of lignite and the degree of freedom is decreasing. Through low-temperature drying,the moisture content in lignite is reduced,the C / H atom is increased, and the high calorific value of coal is increased to a large extent,indicating that the dehydration process has an obvious improving effect on lignite. After dehydration,the mass ratio of fixed carbon to volatile matter and the type of surface functional group of lignite are basically unchanged,indicating that the dehydration process has little effect on the mo- lecular structure of lignite. However,with the increase of dehydration degree,lignite is more easily broken,the average pore diameter is gradually decreased,the BET specific surface area and pore volume are gradually increased,and the extreme values are reached at the boundary between the two stages of low temperature drying. In addition,the removal of moisture can significantly improve the conversion rate and oil yield of lignite in the coal / oil hydro-treating reaction, compared with the lignite without dehydration (The moisture content is 15. 65% ),the conversion of lignite increases by 12. 82% when the moisture content is reduced to 0. 52% ,the oil yield increases by 11. 79% . In this process,the adsorbed water and the non-freezing water are the main removal components,the inter-particle water and free water content and the molecular structure of lignite are basically unchanged,indicating that lignite adsorbed water and non- frozen water are important components that affect the hydrogenation conversion of lignite.