Abstract: The effects of surface microstructure on liquation cracks and fatigue properties of 6005A aluminum alloy MIG joints used in high-speed train bodies were comparatively studied. The results showed that the second phase near the grain boundary of the coarse-grained microstructure was coarse, resulting in larger thickness of the grain boundary liquid film in the heat-affected zone reaching 8~10 μm, which made the liquefied grain boundary less resistant to tensile stress and more sensitive to liquation cracks. The liquation cracks formed by the surface coarse-grained microstructure become possible crack sources during the fatigue process, which adversely affect the fatigue properties of the joints. Improving the surface microstructure of the base metal could suppress the formation of liquation cracks and improve the fatigue performance of the joint. The fatigue strength of the surface coarse-grained joint was 93 MPa under 1 ×10⁷ cycles, while that of the surface fine-grained joint was 107 MPa.