Abstract: A new rapid permittivity measuring method is given through once frequency sweep to obtain the free space reflectivity of concrete plate backed with metal plate. The permittivity is calculated from the reflectivity fluctuating repetition in frequency domain. The interference between surface and bottom reflection is the main cause of microwave reflection fluctuating, which was verified with reflectivity measurement under different condition such as different concrete plate depth, backed on metal plate or absorbing material. According to wave interference theory, the permittivity was calculated from material depth, incidence angle and frequency interval, the result coincides with reference literature well. As the exact reflectivity is not a necessary computing parameter, the error caused by echo clutter is minimized, the exact permittivity can be obtained conveniently from once frequency sweep measurement.