A thin metal casing with plastic parts and a manufacturing method thereof are revealed. A thin metal casing already formed having on one surface processed and coated with a decorative outer layer is provided. A thermoplastic rubber (TPR) adhesive layer is disposed on an unprocessed surface of the thin metal casing. The TPR adhesive layer is synthesized by butadiene-styrene block copolymer and hydrocarbon petroleum resin. Then the thin metal casing with the TPR adhesive layer is set and fixed in a cavity of an injection molding mold. Next plastic parts are formed on the TPR adhesive layer of the thin metal casing by plastic injection molding. The plastic parts are melted and connected to the TPR adhesive layer integrally by pressure and temperature of injection material. Simultaneously a part of the TPR adhesive layer is infiltrated into existing micro concave holes on surface of the thin metal casing.