A Phase-matched filter is applied to extract phase of fundamental surface wave, and then time-variable filter is used to correct amplitude spectrum in order to isolate fundamental mode signs. By processing of synthetic seismogram of surface wave, it is shown that the method can pre-cisely isolate fundamental surface wave from higher synthetic seismogram including the fifth mode, and deformation of amplitude spectrum is very small comparing with spectrum of funda-mental mode resulted from the same model. Based on long period surface wave recording of the Chinese Digital Seismological Network, the fundamental mode is isolated from Rayleigh wave recorded at Beijing and Hailaer stations which is posisioned on the same great circle path with epi-center, the interstation Green's function is estimated from deconvolution of the fundamental mode signs of the two stations. The interstation group velocity and attenuation in a period of from 13 seconds to 140 seconds are calculated from the Green's function, moreover, the results are signifi-cantly more stable.