Abstract: The effective way for detection of the position and the extent of spontaneous fire in underground seams is to measure the radon on the surface. Aiming at the insuffisance that there are restriction of specific conditions and the geochemical significance is not clear in radon measurement on the surface, through the analysis of underground coal combustion data collected by radon measurement on the surface, the paper uses the trational method, the content-sum method of the fractal theory and content-perimeter to determine the bottom limit of radon anomalies, Through comparison, the bottom limit of anomalies was determined, and different extent of fire zone was located. It was found that the the fire extent located by fractal method was more reasonable.