A composite casing structure is applied to a housing of an electronic device. The composite casing structure includes a plastic frame and a metal plate. The plastic frame has a plurality of side walls surrounding four side edges thereof, and at least one open portion is formed by the side walls. The metal plate is combined with the plastic frame, and at least one reinforcing rib is formed on a surface of the metal plate, such that a position of the reinforcing rib is corresponding to a position of the open portion. With such a structural design, the composite casing has a lightweight and high-strength structure.