Abstract：When GPR (ground penetrating radar) is used to detect the distribution of grouting behind the segment of shield tunnel, the determination of the velocity of the electromagnetic wave in the medium is one of the key techniques to ensure the detecting precision and image identification. The coaxial probe method with a network analytic instrument is introduced to test the dielectric constant of the undisturbed soil samples, which are taken in the soil layers (from the third layer to the seventh layer), through which almost all shield tunnels are constructed in Shanghai. The water content and dielectric constant of the samples are tested under different frequencies, and the velocity of electromagnetic wave is computed by use of the dielectric constant. Some comparisons and directional deduction analyses on obtained images are done, and the relationship between the water content and the dielectric constant and that between the dielectric constant and the variation of soil strata are known. All these provide a basis for the GPR detection on the distribution of grouting behind segments.