This　work　reports　a　new　second-order　nonlinear　optical　(NLO)　material　[C(NH2)(3)](3)C3N3S3　(GU(3)TMT),　consisting　of　pi-conjugated　planar　(C3N3S3)(3-)　and　triangular　[C(NH2)(3)](+)　groups.　Interestingly,　GU(3)TMT　exhibits　a　large　NLO　response　(2.0xKH(2)PO(4))　and　moderate　birefringence　0.067　at　wavelength　550　nm,　even　though　(C3N3S3)(3-)　and　[C(NH2)(3)](+)　do　not　exhibit　the　most　favorable　arrangement　in　the　structure　of　GU(3)TMT.　First-principles　calculations　suggest　that　NLO　properties　mainly　originate　from　the　highly　pi-conjugated　(C3N3S3)(3-)　rings,　and　the　pi-conjugated　[C(NH2)(3)](+)　triangles　contribute　much　less　to　the　overall　NLO　response.　This　work　will　inspire　new　thoughts　with　in-depth　on　the　role　of　pi-conjugated　groups　in　NLO　crystals.