Abstract: The crux and application foundation of alpha detecting technique are discussed in this paper. By using this technique, hidden structures can be quickly detected in underground, and the position of hidden faults and the range of hidden sink-hole can be detected on surface too. The test results have proved that this technique is a quick, flexible and economical radioactivity geophysical prospecting tool, it can be playing a more role in coalmine geology. But because of the instrument, etc. , this technique has some limitations in its application.