Taking health diagnosis of Nanjing Gulou tunnel as background, the principles and methods of making non-destructive testing on the tunnel lining quality by using geological radar were expounded. General rules on the radar images, the thickness of formwork concrete and sprayed concrete, metallic doors and windows, the dense dripping section and the leaking water cranny were interpreted. The detecting results of geological radar show that bugs existing behind the tunnel lining is serious with a consecutive leaking water cranny every 5~7 meters, and the concrete thickness is uneven from 27cm to 37cm. When the tunnel is repaired, it is found that the chiseled structure state and the geological radar detecting results are unanimous.