Abstract: Friction Stir Welding is a new and developing welding technology.With the vertical milling machine equiped with auxiliary fixture,a lot of technological experiments on Friction Stir Welding of LY12 alloy had been done,and some theoretic information about the joint figuration was found.Preceding to the next step,the mechanism of Friction Stir Welding and the influence of the stirrer's shape,rotation speed,and welding speed on joint quality were studied.The experiment result shows that when the thickness of LY12 alloy is 4mm,the diameter rate of rotation tool shoulder and the welding pin with special profile is 3:1,the rate of the diameter of the pin and the thickness of welded material is 1:1,the welding speed is 37.5 mm/min and the rotation speed is 2000 r/min,a joint of high quality is formed.The result has laid a theoretical and experimental foundation for further research for Friction Stir Welding.