Abstract: Objective To meet the demand for lightweight, high-performance structural components in aerospace, rail transit, and other fields, aluminum alloys are widely used due to their light weight and high strength. However, friction stir welding (FSW) of thick aluminum alloy plates faces challenges such as uneven weld microstructure, poor material flow, and susceptibility to lack-of-penetration defects, which limit welding efficiency and application. Methods To address these issues, this study optimized FSW with gas tungsten arc (GTA) offset preheating technology. By adjusting preheating position, temperature field distribution between advancing side and retreating side of weld was improved, enhancing metal flowability of weld. An experimental system based on GTA offset preheating was designed to conduct welding experiments on 6 mm thick 2219 aluminum alloy plates. The study systematically analyzed effects of preheating on temperature field distribution, welding load, and metal flowability. Through the combination of thermocouple temperature measurements and data collection from displacement sensors, applicability of GTA preheating model was validated. Results The results show that GTA preheating position significantly influences temperature field distribution and weld performance. When preheating position is located at AS4 on the advancing side of weld, it effectively reduces temperature difference between the two sides, improves the uniformity of temperature field, decreases welding load, and enhances weld quality. Conclusion This study provides a novel approach for FSW of thick aluminum alloy plates, contributing to improving welding process efficiency and joint performance, and promoting further application of FSW technology in manufacturing.