Taking 6061-T6 aluminum alloy used in undercarriage floor of maglev train as the research object,the research is focus on the fatigue strength of 6061-T6 aluminum alloy laser-MIG composite welding joint at low-high tempera ture（-40℃,room temperature,70℃）,provide basic experimental data for maglev train design under wide environmental cond-itions. The result shows that,the fatigue limit at room temperature is 67.1 MPa,weld fracture has obvious labor source zone,fatigue expansion zone and instantaneous fracture zone;the fatigue extension zone has obvious fatigue striation,local fatigue fray of different clusters;the instantaneous fracture zone has obvious dimple structure,and the instantaneous fault zone is ductile fracture. The fatigue limit of low temperatur e（-40 ℃）condition is 61.3 MPa. The fatigue crack of the weld fracture is fine and dense,and there are a large number of secondary cracks, and the weld ductility is reduced. The fatigue limit of high temperature（70 ℃） condition is 56.2 MPa,and the fatigue fracture of weld fracture is characterized by quasi-cleavage fracture. A large number of brittle fatigue ridges are distributed in the fracture and the transient zone is a typical aggregated dimple.