The invention provides a novel self-rotating large-current MAG (Magnetic) welding composite nozzle welding gun for removing slag and a slag removing method thereof. The invention aims at solving the slag removal difficulty of the welding gun in large-current MAG welding process and further optimizing a nozzle structure. The invention mainly comprises a welding gun structure which can remove welding spattering slag in a composite nozzle in a self-rotating way as well as a self-rotating slag-removing method of the welding gun. The welding gun structure and the self-rotating slag-removing method are suitable for thickwire large-current MAG welding.