Abstract: The effects of the taper and the thread number of the stir pin on the plastic metal transfer in the weld transverse section during friction stir welding process were investigated by using aluminum alloy 2024-T4 with the thickness of 20mm. The results show that, the volume of the high temperature metal migrated with the thread of the stirring pin increases with the increase of the thread number on the stir pin when the stir tool is rotated one round. The transfer extents of the plasticized metal in the nugget at the advancing side and retreating side increase in weld thickness direction. That is to say, the height, width and transverse section area of the weld nugget increase in the macroscopic view. When the taper of the stir pin is decreased from 25°to 15°, the amount of the plasticized metal in weld increase, and the transfer extents of plasticized metal on transverse and axial direction increase, and the area of weld nugget also increases. Therefore, when a stir pin with multiple threads or smaller taper is used, the problems of the large temperature gradient at the direction of the plate thickness and the insufficient metal migration in the weld for thick plate FSW can be overcome and the metallurgical quality of the weld can be guaranteed.