Humidity is a key factor affecting the quality of welded joints for high-speed trains. Welded joints made of A7N01S-T5 aluminum alloy were fabricated under five relative environmental humidity conditions: 50%, 60%, 70%, 80%, and 90%. The microstructures of the welded joints were examined using an optical microscope and porosity quantities were calculated from macrographs using image analysis software. The fatigue strength of the welded joints was measured with high-cycle fatigue testing. It was determined that the microstructures and grain sizes in the weld zone and heat-affected zone (HAZ) were similar under different humidity conditions; however, porosity distribution varied significantly. Porosity quantity increased as humidity increased. The weld joint made under the 90% humidity condition had the highest quantity of porosity, while the weld joint made under the 70% humidity condition had the maximum diameter and area of porosity. The weld joint made under the 70% humidity condition also had the lowest fatigue strength. Fracture morphology of fatigue samples showed that the weld joint made under the 70% humidity condition had brittle fracture, while others showed ductile fracture. Therefore, 70% humidity was determined to be the critical humidity level for welding joints in high humidity environment.