A preparation method of a large-size and fine-grain molybdenum tantalum alloy sputtering target material comprises steps of cold isostatic pressing molding through combination of a steel mold with a rubber plate, sintering, levelling and reshaping, hot isostatic pressing treatment, rolling and machining. The grain size of the produced target material is smaller than 50 microns, the change of the grain size is maintained within 20%, and grains are evenly distributed in the plane direction and the thickness direction of the target material; the relative density of the target material is higher than 97%; besides, the large-size sputtering target material with the length being about 2 m and the width being about 1.3 m can be produced.