Abstract: Geophysical methods have been applied for the indirect monitoring of combustion cavity,while exploration through drilling borehole could provide much more information about the combustion cavity,the cavity boundary,and the changes of the roof and bottom rock layer directly. In this study,based on the drilling exploration of a field test of underground coal gasification,the samples of coal,char,ash,slag and rock along the drilling boreholes were obtained, and then the proximate analysis,XRD and SEM-EDS analysis were performed to investigate the changes of their prop- erties. The results show that high activity lignite underground gasification through blowing air,the expanded width of gasification surface is less than 17 m. About 4 m away from the directional drilling channel,perpendicular to the direc- tion of the airflow,high temperature gasification reaction occurs in the coal seam and accompanied by the formation of typical minerals such as anorthite. In this area carbon residue in the ash is about 10% ,and coal seam gasification cau- ses the loose and collapse of overburden. The position 17 m away from the directional drilling channel is close to the boundary of heat transfer,where only the upper coal seam is affected by heat,and the deformation of roof rock is not observed. In addition,the comprehensive profile of the combustion cavity is plotted.