In conclusion, two compounds [C(NH₂)₃]₃Y(SO₄)₃ (I) and [C(NH₂)₃]₂Mg(SO₄)₂(H₂O)₆·2H₂O (II) were successfully synthesized, with their structures consisting of [YO₈]/[MgO₆] polyhedral units, [C(NH₂)₃] cation and [SO₄] tetrahedral group. Experimental results indicate that compounds I and II exhibit SHG intensities of 0.4 and 0.8 times that of KDP, respectively, with their birefringence measured at 550 nm being 0.058 and 0.116. Significantly, compound II shows a lower structural dimensionality, higher birefringence and stronger SHG response than compound I. Further theoretical calculations and structural analyses confirm that the aliovalent replacement of Y³⁺ with Mg²⁺ effectively enhances the material’s birefringence and SHG response. This work presents a novel strategy for preparing high-performance nonlinear optical crystal materials, offering a rational design guidelines for optoelectronic materials.