Abstract: 2 mm thick 5083 aluminum alloy was butt welded by FSW, and defect-free welded joints were obtained. The effect of the welding thermal cycle on the grain structure of the weld was studied by OM and EBSD. The results show that with the increase in the friction stir tool rotation speed, the strain rate rises, and the initial recrystallized grains caused by dynamic recrystallization decrease. The peak welding temperature exhibits a significant increase from 285 °C to 421 °C, accompanied by a corresponding reduction in cooling rate from 32.3 to 20.2 °C/s. This change results in significant grain growth, increasing the average grain size at the weld center from 1.4 to 8.2 μm. Under the high heat input condition, the grains with Goss type 110<001> recrystallized texture grow rapidly, resulting in the shear texture changing to recrystallized texture. The change in the welding thermal cycle does not significantly affect the grain structure and textural type of the weld, and it just affects the final grain size and the proportion of large-angle grain boundaries. This microstructural stability of the weld is attributed to the fact that the grain refinement is dominated by continuous dynamic recrystallization throughout a wide range of heat inputs.