The 6005A-T6 aluminum alloys BT-FSW butt-joints of different surface state before welding were studied respectively through pulsating tensile fatigue test and scanning analysis for the fatigue fractures,the results of fracture analysis showed that:the fatigue limit strength of the polished joint before welding is slightly higher;the welding joints’ fatigue limit strength have no influence by the different surface state before welding. The scanning photos showed that,most of the broken specimens in the fatigue test were broken beginning at the heat-affected zone（on the edge of nugget zone）. The panorama of the fracture was like fibrous,not appearing obvious defects;initiation area has clear source of fatigue with glossy and smooth fracture;plastic fatigue bands parallel to each other can be observed in the expansion zone. The fracture surface of the final fracture area is ductile fracture,showing obvious equiaxed dimple type.