Abstract: The rise of additive manufacturing technology stimulates researchers' enthusiasm for structural innovative design. However, the anisotropy of additive manufactured materials poses certain difficulties for the prediction and design of structural mechanical properties. To accurately predict the elastic properties of polylactic acid (PLA) materials and lattice structures made in fused filament fabrication and realize the elastically isotropic design of lattice structures, firstly, this paper adopts an orthogonal anisotropic elastic model to describe the elastic behavior of the PLA materials, and obtains the nine independent elastic constants needed for the orthogonal anisotropic model through experiments and calculations. Then, a 2D compound truss lattice structure with tunable mechanical properties is designed, and its analytical expressions for the in-plane effective elastic properties and elastic isotropy condition are derived based on the representative volume element (RVE) method without considering the material anisotropy. Finally, the elastic modulus and thickness of the struts in the lattice structure are adjusted according to the anisotropy of the PLA material, and the analytical expressions of the in-plane elastic properties and elastic isotropy condition of the lattice structur are derived based on the RVE method. The results show that the orthogonal anisotropic elastic model is suitable for describing the elastic behavior of fused filament fabricated PLA materials, and the elastic modulus of PLA materials in arbitrary direction can be accurately predicted based on this model. The anisotropy of the material needs to be fully considered when predicting and designing the mechanical properties of the fused filament fabricated lattice structures. After considering the material anisotropy, the elastic isotropy design of part of lattice structures can be realized by adjusting their geometric sizes based on the RVE method.