Generally speaking, porosity defect is one of the inevitable challenges encountered in the joints of aluminum alloy structures during fabrication of high speed railway vehicles. X-ray inspection was used to investigate the porosity rate of aluminum alloy 6082 joints and 5083 joints welded by metal inert gas arc welding (MIG) under different temperatures and humidity. The results demonstrated that the susceptibility of aluminum alloy 6082 welded joints to porosity was higher than that of 5083 welded joints. The porosity rate in aluminum alloy joints was almost determined by the absolute humidity in the welding environment. During the tensile strength test, the fractures of the welded aluminum alloy 6082 joints were mainly located in the heat affected zone (HAZ). With the increase of the absolute humidity, the tensile strength and elongation of the 6082 joints remained almost stable. However, the front and back bending angles of the joints were reduced by 74.4% and 64.4%, respectively. The fractures of the welded aluminum alloy 5083 joints were mainly located in the fusion zone. With the increase of the absolute humidity, the tensile strength and elongation of the 5083 joints decreased by 4.0% and 15.7%, respectively. Whereas, the bending capacity of the 5083 joints was hardly affected by the humidity variation in the welding environment.