Relative permittivity calibration is an important part of ground penetrating radar (GPR) test and detection, which directly affects the test accuracy of GPR. Conventional core-drilling method uses reflection time and the thickness of core to get wave velocity and approximate permittivity, and often ignores the influence of the conductivity on the radar wave velocity. In order to reduce the errors caused by the single core-drilling method, we used the time domain finite difference method to carry out numerical experiments on the double dielectric parameters of reinforced concrete with different conductivities, investigated the relationship among the relative permittivity, the conductivity and the radar wave velocity of the reinforced concrete from the three aspects of the simulation sections, the single scan signal and the snapshots of E_z component respectively, and obtained the correlation between the double dielectric parameters and the radar wave velocity calibration with cubic interpolation and biharmonic interpolation. Results show the correlation between the double dielectric parameters and the radar wave velocity calibration is a section surface distribution and the conductivity and relative dielectric parameters should to be integrally taken into account in radar wave velocity calibration.