Birefringent materials are of great important as opto-electronic functional devices in solid laser system. Investigations on the design of new birefringent materials with large birefringence is meaningful. In this study, structure-birefringence relationship between AgGaS2 (AGS) and NaGaS2 (NGS) was systematically investigated. In the structures, the high ionic Na+ cations broke the three dimensional (3D) [GaS] network to 2D layers, which results in the regular packing of GaS4 tetrahedra and further the birefringence (similar to 0.09, nearly twice that of AGS at similar to 1064 nm). Meanwhile, the blue-shift of the UV cutoff edge to 307 nm and a wide band gap of 3.9 eV has been observed in NGS. The property characterizations were carried out using the as-grown NGS single crystal with a size of 4 x 4 x 0.3 mm(3) by high temperature flux method. The results indicate that NGS may be a promising IR birefringent material and the cation substitution-oriented design strategy could be used to regulate the structure and optical properties of materials.