Abstract: The mechanism of the low?frequency shadows is explained from seismic wave attenuation with instantaneous spectral analysis method and two equivalent formation models are developed in this paper. The horizontal layered medium model analysis shows that in the instantaneous frequency spectrum profiles for the low frequency (such as 15 Hz), the strong energy group occurs beneath the oil?gas bearing layer; when the frequency is above 50 Hz, the energy below the reservoir is much weaker than the energy of the corresponding position of it, which is a typical phenomenon of low?frequency shadows. The simplified caved carbonate reservoir model analysis result indicates that when hydrocarbon is contained in the karst caves, the obvious low?frequency shadows phenomenon may appear near the interface with the wave impedance difference at first beneath the caves, while water is contained in the caves, such a phenomenon is less clear. It is proved that this conclusion is relevant to the real seismic interpretation results, and can be as theoretical basis for identification of oil and water bearing layers by means of the low?frequency shadows