Abstract: Through experiments, this paper validates the modulation on electromagnetic waves generated by time variation plasmas,and proves that the frequency of the modulation keeps consistent with the variation frequency of the plasmas. The essence of this modulation is because the wave vector changes after the electromagnetic waves enter the plasmas, which induces the changes on phases and amplitudes of the electromagnetic waves, and at this time, the plasma is similar to a dispersive medium. The influences of plasmas modulation weakens with the signal frequency increasing. After the demodulation of the amplitude and the phase of the modulated signals, it verifies that the amplitude modulation signals and the phase modulation signals have the same period, but vary differently in the period.