Surface defects refer to the local undulation of the parts in the forming process. The variable cross-section and variable-curvature aluminum trim parts are prone to local surface defects during the stretch-bending process, which has a great impact on the appearance of the whole vehicle. Research on the formation mechanism of surface defects has important guiding significance for the prevention and control of surface defects. In this paper, the formation mechanism of the deformation defects of the stretch-bending forming surface of aluminum trim is studied, the stretch-bending forming test and the surface defects measurement are conducted, and the simulation model of the stretch-bending forming of aluminum trim is established by using the Abaqus software platform. In addition, the forming process of surface defects is analyzed. The results show that when the ratio of section width to bending radius is larger than 35%, the part will have a warping tendency, and the joint action of section shape, upper die, and padding will cause local surface defects.