Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap ( E g ≥ 3.0 eV) and large NLO effect ( d ij ≥ 0.5 × AgGaS 2 ) and simultaneously enlarge the birefringence (Δ n ) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa 3 S 6 O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa 3 S 6 O satisfy the property-balance demand ( E g : 3.21-3.27 eV and d ij : 0.9-1.0 × AgGaS 2 ) as promising PM NLO crystals through gathering their property advantages between LaMGa 3 O 7 and LaMGa 3 S 7 by mixed-anion-oriented performance engineering. A study on the structure-property relationship indicates that heteroleptic (Ln/M)S 7 O and GaS 3 O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.