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In this paper, the friction stir welding (FSW) of single-curvature and double-curvature "S" curved surface lap joints were realized using split type pressing blocks xture and robotic system with welding parameter 1400rpm, 150mm/min. By Finite Element Method (FEM) and axial force analysis, the effects of curvature direction on friction stir welding lap joint of 5083 Aluminum Alloy "S" Curved surface is explored. The results show that Z-axis curvature direction has important in uence on joint forming. By split type pressing blocks xture, the material thermal expansion causes upward force F' in Z-axis positive curvature position, but causes downward force F' in Z-axis negative curvature position. So Z-axis positive curvature makes the actual axial compressive force F R (F R =F N -F') smaller than the device input F N . Which decreases the interface stress of the lap joint and increases the tendency of upper plate edges bending deformation and the probability of interface defects. Finally, the joint mechanical properties are reduced, the tensile load can be as low as 1.5 kN. While Z-axis negative curvature makes the actual axial compressive force F R (F R =F N +F') greater than the device input F N . Which increases the interface stress of the lap joint. Meanwhile it decreases the tendency of upper plate edges bending deformation and leads to the interface defects disappearing. So the joint mechanical properties are good, the tensile load can be more than 2.4 kN. For Y-axis curvature direction, its impact is mainly shown in changing the direction of resultant force F R . In Y-axis positive curvature position, the force F R lies between Y positive direction and Z negative direction. While in Y-axis negative curvature position, the force F R lies between Y negative direction and Z negative direction. The inclination of force F R is helpful to eliminate the interface defects of the joint. At the same time, it will cause inconsistent stress between the advancing side (AS) and the retreating side (RS) of the joint interface, which is the reason that the double-curvature lap joint welding process is unstable.

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