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Benefiting from the planar π-conjugated (HxC3N3O3)x−3 (x = 0–3) groups, cyanurate crystals have recently become a research hotspot in birefringent materials. Herein, by combining the (HxC3N3O3)x−3 (x = 0–3) group with the (CN3H6)+ cationic group, two metal-free cyanurates, GU(H2C3N3O3) (I) and GU3(H2C3N3O3)3(H3C3N3O3) (II), were obtained by the hydrothermal method. These compounds have wide band gaps (∼5 eV) and a large birefringence (∼0.40@400 nm), demonstrating their potential to be ultraviolet birefringent crystals. Moreover, first-principles calculations indicate that their large birefringence values originated from the synergistic effect of the (CN3H6)+ cations and (HxC3N3O3)x−3 (x = 0–3) groups. These findings provide a new design strategy for exploring low-cost UV birefringent crystals with a large birefringence. [ABSTRACT FROM AUTHOR]