Abstract: Based on the observations and quantitative analyses of macerals in 29 coal samples,combining with the analysis of chloroform bitumen "A" group component and gas chromatographical analysis of saturated hydrocarbon,the coal facies properties of mega coal seam in Neogene Miocene Xiaolongtan Formation(N_1x) in Xianfeng Basin are studied.It is indicated that the peat swamp underwent a larger variation of water depth from shallower to deeper and then from deeper to shallower during the formation period of this mega coal seam,totally,the swamp belonged to the low lying one.This mega coal seam had a mixing biological source in which the higher terrestrial plant was the dominant one and the higher aquatic plant was the subsidiary one.Due to the favorable meteorological and hydrological conditions,better accumulating situation and abundant coal-forming original material in the intermont basin,the mega coal seam was formed finally in the stably subsided peat swamp setting in early period of Miocene.