The mechanical properties of fused deposition modeling (FDM) are restricted by natural anisotropy, which hinders its wide application in fields such as aerospace engineering, where there are high strength/stiffness-to-weight ratio demands. Once the internal structure has been determined, the layer thickness and the filling mode would determine the mechanical properties of the printed part. However, the essence of the filling modes provided by commercial slicing software is based on robustness, manufacturing speed, and feasibility, without taking into consideration the high strength/stiffness-to-weight ratio of the parts. In order to reduce the weight and improve the mechanical properties of the printed parts, a lattice-like filling mode with grid used as the main body and inserted circles was designed, and the relevant filling algorithm was developed. Simulations have been performed to compare the capabilities of several parameter combinations, and subsequent mechanical tests have demonstrated that the tensile and compressive moduli of the printed parts filled by the proposed method are respectively about 43% and 23% higher than those of the equal-weight control parts filled by commercial software. This illustrates that the filling mode and corresponding parameter settings can reduce material consumption and improve mechanical performance.