A method includes steps of: preparing first and second plates, each first plate having surrounding apertures, each aperture including a first accommodation portion and an outer necking portion, each second plate having surrounding accommodation portions; stacking the first plates by aligning necking portions to form band gaps extending linearly and parallel to each other; stacking two second plates to two opposing terminal portions of the stacked first plates; penetrating metal bars into the aligned first and second accommodation portions by exposing both ends of each metal bar; beetling the necking portions at each band gap so as to locate firmly the corresponding metal bar; positioning a first mold and a second mold to clamp the stacked plate structure; injecting metal slurries into the first and second molds and then solidifying the metal slurries to form two end rings; and, removing the first and second molds to complete a rotor structure.