The invention discloses a wall-mounted type steel support platform for constructing air-conditioning plates on external walls and a method for applying the wall-mounted type steel support platform. The wall-mounted type steel support platform comprises wall-mounted units. The wall-mounted units comprise vertical rods, first transverse rods, second transverse rods, third transverse rods, first nuts, first screws and second screws. The vertical rods are fixedly connected with formwork systems of the external walls when the wall-mounted type steel support platform is used; the first nuts are fixed onto the second transverse rods; one end of each first screw is screwed into the corresponding first nut, and the other end of each first screw is inserted into a through hole of the corresponding first transverse rod and is connected with the first transverse rod by a second nut; one end of the second screw is connected with the first wall-mounted unit, and the other end of each second screw is connected with the second wall-mounted unit. The wall-mounted type steel support platform and the method have the advantages that the difficult problem of difficulty in supporting formworks of existing air-conditioning plates due to compound apartments, design modification, house type alternation and the like can be solved; construction conditions that internal support frames and external scaffolds are used with one another in the traditional floating plate methods can be reduced; the construction appearance quality and the section sizes of the air-conditioning plates can be guaranteed, and labor, material resources and financial resources for repairing quality defects during decoration can be reduced.