The invention is a Mg-based foam composite and its preparation, concretely a porous Mg-based alloy and its preparing method, and its characteristic: it is a composite prepared by melting process, where the bulk percent of 1-10 mum pores distributed in the metal base is 45%-80%, and its advantages: its preparation is simple, low-cost and no-environmental pollution, its weight is lower than that of the foamed aluminum, and it can act as vibration absorbing parts in rapid machineries like automobiles, etc, and also be used in the fields of sound insulating, noise preventing, etc.