The fatigue property of Al-5Zn-2Mg aluminum alloy Metal-Inert Gas (MIG) welding joints used in high-speed train are studied by fatigue machine. Scanning electron microscopy (SEM) is used to observe the fatigue fracture and the surface damage. Results shows that uniaxial ratcheting behavior take place in the welding zone (WZ) when the stress is higher than 0.5σb. There is obvious plastic deformation in the WZ and a lot of slip band on the surface, which reduces the fatigue life of the sample. The uniaxial ratcheting behavior take place mainly in the first cycle and the finally failure stage of the fatigue process. When the stress is lower than 0.5σb, the fatigue samples crack in the heat affected zone (HAZ). The fatigue fracture of the WZ and the fusion zone (FZ) is typical ductile fracture pattern while that of the HAZ and the base metal (BM) is quasi-cleavage fracture. The fatigue life of the BM is much longer than the HAZ.