The laser particle size analyzer, NMR, SEM/EDS, induction time tests and surface roughness analysis were employed in the mechanism analysis of this investigation. The floatability of lignite was decreased with the pre-wetting increasing from 0 - 5 min because the pores and fractures of lignite surface can be filled with water. However, the floatability of lignite was increased with the pre-wetting from 5 - 480 min. NMR and SEM indicate that a long pre-wetting process (>5 min) polished and rubbed the surface of lignite reducing the amount of macropores and fractures as well as the porosity. The rough lignite surface was also polished to be smooth. As the pre-wetting time increases, the topography of lignite surface, such as porosity, macropores, fractures and roughness decreased, which results in the decrease of water filling negative effects, and hence the floatability of lignite gradually increased with the pre-wetting time over 5 min. (C) 2019 Elsevier B.V. All rights reserved.