Large birefringence (Δn = 0.277 at 546 nm) and strong second-harmonic generation (SHG) intensity (4.3 × KDP) achieved a remarkable balance in a potential ultraviolet nonlinear optical crystal: 2(C3H7N6)+·2Cl-·H2O. Theoretical calculations showed that planar benzene-like π-conjugated [C3N6] groups had important optical properties.