A metallic housing forming method includes steps as follows: friction stir welding metallic members to form a metallic housing; immersing the metallic housing in an alkali solution to remove contaminants from the metallic housing, and forming a black salt layer on the metallic housing; stripping the black salt layer away from the metallic housing; electro-polishing the metallic housing to dissolve the outermost surface of the metallic housing; anode-oxiding the metallic housing; and sealing and drying the metallic housing.