The invention provides an argon tungsten-arc welding and cold metal transition welding composite heat source welding device and a method and application. A TIG welding gun and a CMT welding gun are fixed in the same horizontal plane; the TIG welding gun is installed in front of the CMT welding gun along the welding direction, and the TIG welding gun and the CMT welding gun are perpendicular to the workpiece surface; the distance between the TIG welding gun and the CMT welding gun is 20-70 mm; and the shielding gases of the TIG welding gun and the CMT welding gun are one of nitrogen, helium and argon. The method is capable of solving the problem that the liquid state molten bath spreadability of the pure CMT welding is not good, improving the welding line forming, increasing the depth of fusion, realizing the plating thickness range of the once forming of the metal plate butting, improving the welding efficiency, enlarging the application range of the CMT and TIG in the moderate thickness plate welding, improving the fine texture of the welded joint, and improving the welding quality.