The wavele t is confirmed by modulation frequency, energy attenuation factor of wavelet, th e amplitude and phase of wavelet in seismic theory. Drawing lessons from this th eory in this paper, the Q value is set as a constant in each layer of horiz ontal ly layered medium (Q value is independent of frequency). Firstly, the freque n cy domain equation of the single pass propagation theory of plane wave is solve d . Through the definition of envelope peak instantaneous frequency and the deriva tion of equation, it is shown that Q value is confirmed by the changes of en velope peak instanta neous frequency, the travel time and the wavelet energy attenuation factor. So t he m ethod to estimate Q value using envelope peak instantaneous frequency of VRP(vertical radar profiling) data direct wave of ground penetrating radar is obtained. At last, the effectiveness a nd accuracy of the method are verified using synthetic VRP data, synthesis experimental data and practical data.