This paper deals with the basic principle of the radon measuring method for detecting goaf. The authors measured the γ radial intensity of radon daughter in the adsorber so as to understand the scale of radon thickness and hence get to know the position of the coal mine goaf. On the basis of geophysical characteristics of goaf and by using the transient electromagnetic method and analyzing the apparent resistivity section, the authors accurately located the goaf. Exemplified by detecting the goaf of the Dongqu mine affiliated to the Xishan Mine Electricity Group Corporation of Shanxi Province, this paper holds that the radon measurement and transient electromagnetic methods can be used to explore the coal mine goaf.