Abstract: Radon detection technique can effectively delineate the extent of combustion space of underground coal gasification.In order to accurately delineate the position and range of radon anomaly,the authors used trend surface analysis,employed different degrees of polynomial to fit the overall distribution trend of radon element,based on the fitting analysis and F distribution test,determined the degree of polynomial of the best trend surface used to delineate radon anomaly,and then analyzed the residual (surplus),recognized local anomalies and ultimately delineated the range of radon anomaly.Exemplified by the radon data obtained in the combustion space area (CSA) of underground coal gasification (UCG) in Wulanchabu,Inner Mongolia,this paper has validated the feasibility of the method in delineating the CSA of underground coal gasification.Studies show that the radon anomalies delineated by this method is obviously superior to the traditional method,coincides well with the verified drilling points and this method is rapid,simple and effective,can be applied in low radioactive area.