Design of functional materials with targeted properties is a challenge because of the diversity of their potential structures. The functional performances of materials are mainly determined by the chemistry and electronic structure of modules consisting of local atomic groups with special arrangements. Tetrahedral modules are excellent modules for designing deep-ultraviolet/ultraviolet (UV) nonlinear optical (NLO) materials, but they are rarely favored due to their unpredictable optical anisotropy and second harmonic generation (SHG) response. In this work, we have developed a module-guided ab initio approach for evaluating the optical anisotropy of tetrahedral modules. The application of this method indicates that the tetrahedral modules with a specific arrangement will enhance the optical anisotropy of materials. With the functional modules consisting of tetrahedral modules and rare-earth cations, new high-performance rare-earth phosphates were assembled. These materials are promising deep-UV NLO materials because of their appropriate birefringences, large band gaps, moderate SHG responses, and easy to obtain large size crystals.