<p>Triangular defects generated during the Automated Fiber Placement (AFP) process in double-curvature composite structures are a common flaw that significantly affects structural mechanical properties. This study systematically investigates the formation mechanisms of triangular defects, examines their various geometries, and analyzes their stacking behavior across different plies in relation to mechanical performance. The goal is to determine the AFP trajectory design strategy to balance the processability of layup and the performance of the structure for the engineering products. The findings indicate that defect geometry has a relatively minor effect on specimen performance. However, the number of defects and their overlap considerably influence structural behavior. Specifically, a 0° triangular defect in the first ply reduces tensile properties by 13.5%, whereas a 3° defect in the same ply decreases tensile performance by 17.5%. Notably, the presence of two overlapping 3° defects results in a 33.5% reduction in tensile properties, and three overlapping 3° defects lead to a 41.3% decline. Consequently, the design of AFP trajectories for composite structures should aim to minimize the occurrence of triangular defects. Moreover, defects in the same orientation across different plies should be staggered to avoid overlap in the thickness direction, thereby reducing the risk of structural failure. In addition, finite element (FE) simulation analysis demonstrates that the deviation between the triangular defect model and experimental results remains within ± 8%, with deviations for single-ply defects below 5%. This simple FE model and test method are considered reliable and can be utilized for trajectory design, triangular defect analysis, and the control of triangular defects in large size composite structures, thereby enhancing defect management in AFP, and promote the engineering project forward.</p>