Abstract: The effects of needle length and weld position on microstructure and properties of 25 mm thick 6082-T6 aluminum alloy double-sided friction stir welding were studied in this paper. The results show that when the length of the stirring needle is not enough to cover half the thickness of the plate, the stirring force at the end of the stirring needle is insufficient, and there exist large defect in the weld center, and the tensile and bending properties are poor. When the length of the stirring needle is just over 1/2 plate thickness, there is still a small defect in the weld center, but it does not affect the tensile and bending properties obviously. When the length of the stirring needle continues to increase, the defects in the center of the weld are eliminated and the bending performance is good, but the joint strength decreases with the needle length continues to increase. In this study, the joint comprehensive performance is the best when the needle length is 14 mm. At the same time, it is found that the welding direction and the lateral offset direction affect the bending performance of the joint. When the reverse weld is offset by 2.5 mm to the retreat side, the bending sample is intact. When the reverse weld is offset by 2.5 mm to the advanced side, the backward HAZ with weaker performance will be close to the weld center, leading to the cracking of the bending sample.