The corrosion fatigue characteristics of the Al-Mg-Si alloy and its butt welded joints in a simulated industrial atmosphere are studied by means of mechanical performance test, axial force loading fatigue test, scanning electron microscope, electrochemical test and other methods. The results show that the mechanical properties of butt-welded joints of Al-Mg-Si alloy are lower than that of the base metal, and the welded joints are more prone to corrosion in the simulated industrial atmosphere. The corrosion fatigue sensitivity of welded joints is higher, the corrosion fatigue cracks of the base metal initiate from corrosion pits at grain boundaries, while the fatigue cracks of the welded joints initiate easily at the locations rich in welding defects and inclusions of the weld seam. The existence of large number of welding defects and Al₂O₃ inclusions introduced during the welding process may be responsible to why the corrosion fatigue performance of the welded joints of Al-Mg-Si alloy is lower than that of the base metal. Besides, the presence of inclusions will induce stress concentration, and around which high-density lattice distortion areas will also emerge. These areas will act as anodes to be preferentially dissolved and thereby, where to become the priority area of corrosion fatigue crack initiation, eventually lead to fatigue fracture of the test piece. Therefore, for the actual production, the forming process and welding process should be optimized, and the number of welding defects and inclusions should be controlled, so as to improve the corrosion fatigue performance of the alloy.