Abstract: In bobbin tool friction stir welding (BT-FSW), the simultaneous introduction of upper and lower shoulders leads to a material flow behaviour that significantly deviates from that in conventional friction stir welding (FSW). To reveal the effect of rotation speed on material flow behaviour during BT-FSW, BT-FSW experiments on 6 mm thick 2219-T8 aluminum alloy plates at two rotation speeds were conducted, and a thermal–mechanical–fluid fully coupled simulation model was established based on computational fluid dynamics (CFD). Through the coupled validation of experiment and numerical simulation, the evolution mechanisms of material flow behaviour and deposition process at the two rotation speeds were systematically compared. The results indicate that at a high rotation speed of 500 r/min, the material on the advancing side (AS) exhibits a multi-loop recirculation flow pattern near the shoulder, generates a significant vertical flow, converges toward the plate thickness center, and finally deposits on the AS behind the tool; the material on the retreating side (RS) and around the pin presents a through-flow pattern. At a low rotation speed of 200 r/min, the material flow velocity is low, and the material exhibits a through-flow pattern on both AS and RS.