When we use the ground penetrating radar（GPR） to detect the effect of grouting behind segment at the shield tunnel tail

it is very important to ascertain the velocity of electromagnetic wave（EMW） to obtain accurate results.It is unrealistic to assume that permittivity and electric conductivity are invariable

when the velocity of electromagnetic wave was estimated.In order to assure the precision of GPR image identification

network analytic instrument is firstly introduced to detect the dielectric permittivity of grouting material under the different frequencies

mixture ratios and curing periods.Some direction deduces analyses based on radial tracing theory have been done and compared with the results of the same grouting material on the site.This study shows that it is very necessary to adopt the different dielectric permittivities according to the different frequencies and curing periods.