Abstract: The formation of lazy S weld in the friction stir welding of LY12 aluminum alloy was analyzed by the copper foils as a tracer material in the alloy.The influence of the welding pa-rameters on the formation of lazy S weld was investigated with a conical pin tool.The results show that lazy S weld is formed by the distortion of the original weld interface and its surrounding metal.The shape of the lazy S weld depends on the cooperation of the rotation of the upper part metal in the weld,the interaction between the stir pin and the metal around the original weld inter-face and,the metal at the bottom of the plate translating to the area away from the weld center and to the upper part of the plate.If the tool rotating speed is increased at the constant weld-ing speed or the welding speed is decreased at the given tool rota-ting speed,the lazy S weld covers less area along the weld thick-ness direction and disappears in the end.