Abstract: The left and right thread stirring was used to carry out friction stir welding of 6005-T6 aluminum alloy profiles, and the influence of the thread rotation direction of the stirring needle and the rotation direction of the spindle on the welding defects was studied. A multi-field coupling calculation model of friction stir welding was established by ABAQUS, and the flow of plastic materials in the weld nugget zone and the formation of welding defects in the friction stir welding process were simulated and analyzed. The results show that the simulation results are in good agreement with the test results, and the reliability of the simulation is high for the hole defects inside the weld. Under the condition that the shoulder thread remains unchanged, the right-handed thread stir pin can obtain a high-quality welded joint without welding defects, while the welded joint welded by the left-hand thread stir pin produces a hole defect at the bottom of the joint, and the hole defect at the bottom of the nugget area becomes larger after changing the spindle rotation direction. Numerical calculations were used to predict that the left-handed threaded stirring needle could not obtain a welded joint without hole defects under different welding process conditions.