This paper describes the principle and technique of applying ground penetrating radar to quality inspection of the tunnel lining, and expounds its basic method as well as the data and image processing procedure. The error in inspecting thickness of tunnel lining is within 5%, the quantity of steel rod in the tunnel lining can be determined exactly, and the density of the filling can be estimated roughly. The effect of this technique in inspecting tunnel lining is satisfactory as shown by its application in Laomuyu tunnel.