Underground gasification of coal means the controlled combustion of underground coal.In order to control the combustion and gasification of coal in an effective way,it is quite important to understand the moving velocity of the burning flame working plane and the limits of burned out area.During the test conducted in the underground gasified area of the Liuzhuang coal mine,the authors analysed the feasibility and effctiveness of the radioactive radon technique in the determination of the moving velocity of the buring flame working plane and the limits of the burned out area in the process of coal underground gasification monitoring.On such a basis,some suggestions are put forward in this paper.