Taking the friction stir welded joints of 3 mm thickness 6061-T6 aluminum alloy plate as the research object, a thermal mechanical coupling finite element model was established to accurately simulate the temperature filed distribution and evolution law of the welding process. The influence of different welding speeds on forming characteristics, microstructure and mechanical properties of the joints was studied by using optical microscopy, electron backscatter diffraction, microhardness measurement and tensile test. The results show that a completely dynamic recrystallization process occurs in the nugget zone during the welding process, which generates fine equiaxed grains, whereas the heat-affected zone (HAZ) on the retreating side undergoes dynamic recovery, resulting in the obvious grain growth. It is also found that the strengthening effect of grain boundary in the HAZ is weaker than that in the nugget zone. The joints are well-welded when the welding speed is 300~800 mm/min, and the fracture occurring in the HAZ of the retreating side could be mainly attributed to the temperature（400~480 ℃），dissolution of the precipitation phase leads to a significant reduction in the mechanical properties，and in this welding speed range，the joint strength increases with the increase of welding speed，and the highest strength factor is 80.86%（800 mm/min）. As the welding speed is further increased to 1 200 mm/min, the weld formability of the joint would deteriorate significantly due to the insufficient heat input and thermo-plasticity. Besides, the un-welded and tunnel defects in the weld nugget lead to the occurrence of the fracture in the nugget zone during tensile test.