Abstract: During the tunnel construction, the surrounding circumstances of a tunnel and the geological and hydro-geologic conditions ahead of working surface are very important to tunnel construction in both quality and security. A poor condition can cause tunnel mudslide. Suddenly water leaking not only bring difficulty to tunnel construction but also cause human life in danger, equipment damage and delay the completion of the project. These will cause immense pecuniary loss. Compared with other geophysical exploration equipments, the geological radar has a lot of advantages, such as high resolution, low cost, rapid speed, less construction disturbance, etc. So it has been applied widely in the area of geologic prediction in tunnel. In combination with the tunnel project of the Dahualing, the principle and the detection and interpretation methods of geological radar are introduced, the typical radar images of intact rock mass, swelling rock mass, fractured rock mass and water-bearing rock mass are established and explained in the planning detected region, the characteristics of radar maps are analyzed, and the practicality and efficiency of geological radar is illustrated.