Abstract: The friction stir welding (FSW) process of aluminum alloy liquid cold plate lap with narrow step is studied in this paper. The cold plate material is 6063 aluminum and the cover plate material is 3A21 aluminum. The FSW pin tool for the liquid cold plate with narrow lap is designed, the pin tool shoulder width and welding pressure are reduced and the joint welding fluidity is increased. The FSW process test is carried out for the narrow step cold plates with different welding depth. Study results show that the narrow lap FSW welding of 4-2 (weld depth-step width, mm), 6-4 and 9-6 can be realized by optimizing the dimension of pin tool and the process parameters. Location pre-welding before welding can effectively avoid S-curve of the weld. Good welds can be formed when 6063 aluminum or 3A21 aluminum is used as the forward side.