Abstract: GPR is a kind of physical detecting technology, which has high efficiency, high resolution and non-destruction; it is a useful geophysical detection technique in detecting underground cavity. The cavity underground tunnel can cause disaster such as the road surface l sinking, lining crazing and top floor collapsing, etc., affecting the safety of tunnel. It's important to accurately detect cavity underground tunnel and take corresponding measures to insure the tunnel's safety. According to the process of the coal pits detecting in the tunnels of Mountain Lion, this issue introduces the application of Ground Penetrating Radar to coal kiln in tunnel.