In order to enhance the strength of 3D printed parts made of polymer materials and reduce the anisotropy caused by the fused filament fabrication process, this paper proposes an inter-layer interleaved composite path planning method based on the directional partition of the principal stress field. This method not only ensures intra-layer reinforcement under specific working conditions but also increases the strength of the printed part by enhancing the adhesion of adjacent layers through staggered filling between layers. The mechanical performance is improved by 17% compared to other conventional filling patterns such as the zigzag filling algorithm. IICP is suitable for path planning of structures with complex shapes and is easily integrated into existing general computer-aided engineering processes.