Abstract: The vehicle-mounted GPR technique, a non-contact and long-distance detection method with air-cou? pled antennas, is a fast nondestructive detecting technique in the field of structural health safety assessment of tunnels and underground engineering, which has changed the traditional measurement method of hand-held GPR antenna closely attached to the tunnel lining. This paper describes the detecting conditions and working principles of this technique used in health assessment of metro tunnels, and evaluates the feasibility of vehicle-mounted GPR to detect the metro tunnel in the basis of analyzing the test results. The experimental results show that this technique could detect the metro tunnel at normal operating speed within the vehicle gauge, providing a new method for the maintenance and safety inspection of operating metro tunnels.