The invention provides a preparation method for realizing a warm-molded medium manganese steel piece with performance gradient and equal thickness. The preparation method comprises the following steps: a medium manganese steel plate is selected as a steel plate to be used; after the steel plate to be used is heated to reach complete austenitizing, the steel plate to be used is transferred onto a punching mold for molding by punching, and is quenched to room temperature to obtain a warm-molded steel piece; the cooled steel piece is locally or totally heated in a gradient heating mode to realize gradient change of the heating time of the steel piece in different positions so as to finish cooling of the steel piece to the room temperature after the inverse phase change process, and the medium manganese steel piece with performance gradient distribution, high strength, high plasticity and equal thickness is obtained. The medium manganese steel piece is a multi-phase structure; the same one steel piece realizes ultrahigh strength, and obtains high-plasticity performance gradient distribution; and in high-plasticity area, because of gradient content change of austenite, the performance gradient characteristic is further realized, so that the demands of lightness and safety are satisfied, and the popularization significance is wide.