The invention provides a preparation method of ultrathin Ta-W alloy foil, and belongs to the technical field of Ta-W alloy machining. The preparation method includes the steps that cyclic operation of alloy billet prepared through a powder metallurgy method, cold rolling cogging, and cold rolling and vacuum annealing, 3-5 micrometer foil annealing, and the like. The thickness of the prepared foil can reach 3-5 micrometers. The preparation method is simple in process, the prepared foil is high in precision, and compared with pure Ta foil and other Ta-W alloy foil, the ultrathin Ta-W alloy foil has the advantages of being high in strength, good in surface quality, and the like. The prepared Ta-(5.0-7.5 wt%)W alloy foil with the thickness of 3-5 micrometers is suitable for high-power microwave tubes, traveling wave tubes, and other vacuum devices of electronic components, aerospace, and other industries. The preparation method has great significance on achieving high-power high-performance microwave tube localization, improving microwave tube using performance, prolonging service life, and the like.