Abstract: Numerical simulation of material flow in the process of refill friction stir spot welding was investigated by the CFD software ANSYS FLUENT. Effects of the rotational speed and refill velocity on the material flow behavior were mainly discussed. The simulation results show that the material flow velocity becomes smaller with the increase of the distance away from the inner and outer edge of the shoulder. The peak flow velocity of material appears at the outer edge of the shoulder. Inside the shoulder, the material with the minimum flow velocity locates in the center of welding spot, where the flow velocity can be improved by increasing the rotational speed. In terms of enlarging the effective bonding area, increasing the rotational speed of the tool is more significant than increasing the refill velocity. These conclusions by simulation were verified by the experiment.