Abstract: This study investigates the root cause of leakage failure in the friction stir weld (FSW) of an electric vehicle battery pack lower tray during mass production. It aims to identify the defect, clarify its formation mechanism, and propose effective process improvements.The leakage fault was reproduced and located through helium leak testing. Macro-observation and metallographic examination (OM) were employed to characterize the defect morphology, combined with an analysis of the welding process to determine the cause.The results show that the leakage was attributed to a lack-of-penetration defect, specifically a "cold weld, " caused by severe stirring pin deviation during FSW. This defect, resembling a crack, created a continuous channel for leakage. By implementing a laser tracking system to control pin alignment within ±0.2 mm and optimizing parameters to a rotation speed of 1500 rpm and a travel speed of 800 mm/min, the defect was effectively eliminated. Precise control of stirring pin alignment is critical for preventing such leakage defects. The optimized process, validated through batch production, significantly reduced the leakage rate, achieving a consistent product qualification rate above 99.8% and a joint strength reaching 73.5% of the base material. This provides an effective solution for enhancing the quality and reliability of FSW joints in battery packs.