The mechanical properties of welded joints directly affect the safety and service life of railway vehicles. The 6005A-T6 aluminum alloy butt-welded joint prepared by gas metal arc welding, which is commonly used for rail vehicle car bodies, was taken as the research object, and a study on the local mechanical properties and multi-stress ratio fatigue limit of the welded joint was conducted. Firstly, the micro-Vickers hardness test was employed to delineate the boundaries of the base material, heat-affected zone, and weld metal. Secondly, digital image correlation technology was utilized to obtain the true stress-strain curves of each region, by which their elastic modulus, yield strength, and tensile strength were quantitatively characterized. Finally, the fatigue limit of the joint under stress ratios R=?1, 0.1, and 0.3 was tested using the staircase method, and an appropriate constant life prediction model for the joint was established. The results show that the hardness distribution of the welded joint perpendicular to the welding direction exhibits a “W” shape. The lowest hardness value (approximately 60 HV) is located in the heat-affected zone, representing a 30% reduction compared to the base material. The elastic moduli of the base material, heat-affected zone, and weld metal are similar; however, the yield strength of the weld metal drops by more than 40% compared to the base material. The fatigue strength of the joint is only 20%-33% of its static strength and decreases significantly with increasing mean stress. The linear constant life model established based on experimental data has a prediction error of approximately 3% for the fatigue limit under the condition of R=0.5, demonstrating better accuracy than the classical Goodman and Gerber models. This study can provide data support and a theoretical basis for the anti-fatigue design of 6005A-T6 aluminum alloy welded structures in car bodies of railway vehicles.