Combining the engineering practical experience with theoretical knowledge, this paper has summarized the problems that should be tackled when the ground penetrating radar is employed in such aspects as railway tunnel lining thickness, cavity behind the lining, internal reinforcement and steel frame detection, especially the detection accuracy of the lining thickness, the interpretation of the second rows of steel in the cases of double layers reinforcing and steel frame detection when the lining has been reinforced. This paper aims to show the actual detection capability of the ground penetrating radar at the current level of science and technology with the purpose of helping the engineers and technicians to have a further cognition of the problems that can be solved by radar.