Abstract: The median fatigue limit of 6005 A-T6 aluminum alloy plates with different thicknesses(4 mm, 12 mm, 20 mm and 40 mm) were measured by the staircase method, and the microstructure and fatigue fracture of aluminum alloy plate were analyzed by OM and SEM. The results show that the median fatigue limit of 4 mm thick plate is greater than that of 12 mm thick plate, that the median fatigue limit of 20 mm thick plate is greater than that of 40 mm thick plate. and that the median fatigue limit in the direction of rolling is greater than that in the vertical rolling direction, The main reason is that the size of impurity phase of 4 mm thick plate is smaller and the number less than those of 12 mm thick plate, and the size of impurity phase of 20 mm thick plate is smaller and the number less than those of 40 mm thick plate. Besides, the impurity phase is directionally distributed. Delamination appears on fatigue fractures of all the plates, and with the increase of thickness, the delamination degree of fatigue fracture becomes more and more obvious.