The invention relates to a metal active gas (MAG) welding method for butting quenched low-alloy super-high-strength steel and high-quality carbon structural steel and a low-alloy super-high-strength steel MAG welding method, which solve the welding technical problems that the thickness difference and the material performance difference of heterogeneous steel, namely the quenched low-alloy super-high-strength steel and the high-quality carbon structural steel, are large and the weldability of the quenched super-high-strength steel is low. The MAG welding method comprises the following main steps of: machining a part to be welded; quenching and tempering a 35CrMnSiA barrel piece; preheating, tacking and welding; and slowly cooling the 35CrMnSiA barrel piece; and detecting the 35CrMnSiA barrel piece. By the MAG welding method, the weldability of the quenched super-high-strength steel and the carbon structural steel is enhanced; fusion of a welding line is high; and through penetrant flawdetection and coloring flaw detection, the most sensitive crack defects of the super-high-strength steel are not found in a welding bead and a heat affected zone, so the shape of the welding line is good. The MAG welding method belongs to semi-automatic welding; and compared with manual arc welding, the MAG welding method has the advantages of reducing arc striking points and arc receiving points, so that welding defects can be reduced effectively.