The invention provides a flat butt joint MIG welding method for a dissimilar aluminum alloy welding joint. According to the flat butt joint MIG welding method, a conventional aluminum alloy plate anda high-strength corrosion-resistant aluminum-magnesium alloy plate are subjected to MIG double-sided welding, the welding position is a flat position, a single-side symmetrical V-shaped groove of 14-17 degrees is adopted for a groove, and the gap width of the bottom of the groove is 0-2 mm. The method is particularly suitable for welding between H116-state 5083 aluminum alloy and O-state high-strength corrosion-resistant aluminum-magnesium alloy, a welding seam is uniform in surface forming and is free of obvious surface defects such as cracks and incomplete fusion, compared with a traditionalwelding method, the flat butt joint MIG welding method has fewer post welding air holes, the yield strength and the tensile strength of the welding joint are high, and the special use requirement ofa ship can be met.