Abstract: To improve precision of the interpretation of Ground Penetrating Radar (GPR) images during tunnel inspection and provide interpretation templates for typical images, a 2D forward simulation was carried out regarding such typical detection targets as the tunnel’s reinforcement, steel arch, lattice girder, rock disengagement, cavities, and uncompacted areas based on GPR theory and the finite difference time domain method. The features of the GPR images of these targets are summarized by comparing the simulated results with the measured ones. The results show that the ground penetrating radar can be used to detect reinforcement, steel arches, rock disengagements, cavities and uncompacted areas within tunnels, and the resulting images can be used to provide guidance and reference for later interpretation.