A new type of M?ssbauer spectroscopy-time-dependent M?ssbauer spectroscopy isproposed and realized on the basis of using two-step decay 119mTe→119Sb→119Sn.For the GaAs samples, implanted with dose of 110keV 119mTe + 1015 stable Te/cm2 and annealed at 600℃, the relative intensities of various lines of the M?ssbauer spectra have been measured as functions of ξ, a parameter being equal to ΦTe/ΦTo+a1 by definition ξ itself is determined from the annealing time ta, the initial time ti and the final time tf of the measurement of each spectrum. This kind of time-dependent M?ssbauer spectroscopy can be used to investigate the change of the environment of the probe atoms in solid. The lines of M?ssbauer spectra have been identified. The problem of the environment change of Te atom in GaAs after its decay into Sb and the mechanism of the non-substitutional Te have been discussed. Our experimental results are also compared with previous data taken from M?ssbauer spectra of 125Te and 129Te in GaAs.