Abstract: 4 mm thick 1561 aluminum alloy was friction stir welded by the constant pressure controlling mode. The characteristics of the welding pressure, the microstructures and the mechanical properties of the joints were studied. The results indicated that in the plunging stage, the welding pressure alternately experienced increase, decrease and then increase. In the stable welding stage, the welding pressure changes as a sinusoidal periodic trend induced by the variation of the mechanical property of the welded metal. Fixed the welding speed at 200 mm/min, when the rotation speed is lower than 800 r/min or higher than 1 800 r/min, the void defect formes in the weld. When the rotation speed is higher than 1 000 r/min, the "S" line appeares in the stir zone. The tensile strength of the FSW joint is depended on the material strength of the joint when the rotation speed is relatively low. When the rotation speed is relatively high, the "S" line has significant effect on the tensile strength of the FSW joint. With the increase of the rotation speed, the tensile strength of the joint firs increases and then decreases.