Abstract: An aluminum alloy sheet 2A12-T4 two millimeters thick was welded in friction stir spot welding with re-filling probe hole,and the influence of processing parameters on mechanical properties of joint was studied.The results show that the shearing resistance of the joint reaches the maximum value of 4 707 N when rotational speed is 600 r/min and other parameters keep constant.The shearing resistance of joint decreases with the increasing of rotational speed in range from 600 to 1 000 r/min,increases with the increasing of the distance of shoulder plunged,which reaches maximum when the distance of shoulder plunged is 1.2 mm,and increases with the increasing of extension length of pin-tool,which reaches the maximum when the extension length is 3.4 mm.The micro-hardness of the joint is between 100 to 150 HV,and that in mixed zone is highest and that in stir zone is lowest.