Abstract: In order to analyze the target feature information of GPR time domain transient backscatter response that is focused in recent years, an estimating algorithm for the correct start of the late-time response is proposed in this paper. Basing on that the energy of surface reflection at different polarization is equal in GPR early-time response stage for buried target, the starting point of windowing function is determined. Estimating the pole series by Matrix Pencil Method and using poles convergence of late-time response，the earliest convergence time is the beginning of the late-time response stage. With the example of backscattered response of the slender cylinder buried dry sand environment, the results of numerical simulation and experiments show that this processing method can separate late-time response effectively.