Abstract: During the inspection process of a certain high-speed train, it was found that the connection part of the support beam at the end was corroded, posing a safety hazard to the service life of the vehicle structure. Scanning electron microscope, metallographic microscope, transmission electron microscope, and electrochemical workstation were used to conduct failure analysis from perspectives such as the morphology of the corrosion surface and characteristics of the corrosion products, the progress of corrosion in the base section and microstructure, the aging of the anti-corrosion primer, and the electrochemical properties of the joint metals. The results show that the failure mode of the component is gap corrosion + galvanic corrosion + flaking corrosion. The gap structure formed at the connection parts of support beam end stores mud and water for a longer time, which promotes the hydrolysis and aging of the anti-corrosion primer and the corrosion of base metal. Then the aluminum alloy support beam is connected with the vehicle body using stainless steel angle codes and bolts, and the contact of different materials in the corrosive medium forms galvanic corrosion, accelerating the corrosion rate of the base metals. The aluminum alloy base material has high-density dislocation tangles and coarse grain boundaries along the rolling direction, which dissolve preferentially in the corrosive medium, forming corrosion tunnels inside the base material and leading to flaking corrosion, further promoting the corrosion process. Finally, preventive measures are proposed based on the failure analysis results.