The invention relates to a microwave attenuator material and a preparation method thereof, in particular to an AlN-Mo microwave attenuator material and a preparation method thereof, and belongs to the technical field of microwave electronic vacuum. The microwave attenuator material comprises 20 to 25 volume percent of metal phase Mo, 2 to 5 volume percent of sintering aid CaF2 and the balance of dielectric phase AlN. The preparation method comprises the following steps of: preparing AlN-Mo composite powder by adopting a slurry coating method, and performing unidirectional axial press molding and then performing cold isostatic press molding on the AlN-Mo composite powder to obtain an AlN-Mo composite green blank; and sintering the green blank in a ZrO2 or AlN crucible under nitrogen atmosphere and normal pressure to obtain an AlN-Mo composite block material. The method reduces the sintering temperature, shortens the sintering time, improves the sintering compactness of the metal-medium composite microwave attenuator material, is favorable for batch production, and can obtain the metal-medium composite microwave attenuator material with high compactness, no defect and good uniformity and consistency.