The invention provides a composite shell structure applied to a shell of an electronic device. The composite shell structure comprises a plastic frame part and a metal sheet, wherein a plurality of side walls surround the plastic frame part, thus forming four side edges; at least a permeable part is defined by the side walls; the metal sheet is combined on the plastic frame part; at least a reinforcing rib is formed on the surface of the metal sheet; and the positions of the reinforcing ribs can correspond to the positions of the permeable parts. With such structural design, the composite shell has light and high-strength structure.