[PO4]3−　possesses　small　microscopic　second-order　susceptibility　and　polarizability　anisotropy,　which　inherently　result　in　weak　second-harmonic　generation　(SHG)　and　small　birefringence.　Herein,　a　new　noncentrosymmetric　phosphate,　[C(NH2)3]3PO4·2H2O,　was　successfully　designed　and　synthesized　by　a　solution　evaporation　method.　[C(NH2)3]3PO4·2H2O　without　metal　ions　is　composed　of　planar　conjugated　guanidine　cations　and　PO4　tetrahedrons,　which　are　connected　by　hydrogen　bonds,　forming　a　three-dimensional　network.　Physical　property　measurements　indicate　that　the　title　compound　displays　an　SHG　response　of　1.5　×　KH2PO4　(KDP)　and　a　larger　birefringence　(0.055@546.1　nm)　compared　with　most　reported　ultraviolet　nonlinear　optical　phosphates.　Furthermore,　first-principles　calculations　reveal　that　the　π-conjugated　planar　[C(NH2)3]+　cations　and　[PO4]3−　groups　are　responsible　for　its　excellent　linear　and　NLO　properties.　©　2021,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.