Abstract: Force components serve as the information carriers for defect evolution in aluminum alloy friction stir welding, playing a crucial role in defect prediction and identification. This paper employs a force measuring platform to conduct in-situ measurements of traverse force, lateral force, and plunge force during the welding process. It further analyzes the relationship between force characteristics and the formation and size of defects. The results indicate that welding force initially increases, then stabilizes, and ultimately decreases rapidly during the welding process. Tunnel defects are observed within the weld under conditions of low rotation speed（≤750r/min） or high welding speed（≥240mm/min）. The varitaitons in welding force are primarily characterized by a reduction in plunge force and an increase in lateral force. Notably, the lateral force exhibits a positive correlation with the size of tunnel defects, with a correlation coefficient of 0.92, while the plunge force shows a negative correlation with defect size,with a correlation coefficient of-0.60. Furthermore, the reduction in plunge force is attributed to the diminished extrusion effect of the shoulder on the weld material, making it difficult for the material to be wrapped and migrated to the weld surface, thereby leading to the formation of tunnel and groove defects. The increase in lateral force results from the reduced ability of the stirring pin to drive the material, causing a buildup of material in front of the stirring pin on the retreating side of the weld. This exacerbates the extrusion of materials on the retreating side by the stirring pin, making it challenging for the materials behind the stirring pin to backfill, which ultimately leads to the formation of tunnel defects.