A sodium zinc thiostannate Na_2ZnSn_2S_6 is reported with its interesting structural features and property characteristics. It crystallizes in the polar space %xou^Fddl(No. 43) with the unit cell parameters of a=12.945(3),b=23.556(5), c=7.2774(15)and Z=8. In its structure,Zn-centered penta-tetrahedra [ZnSn_4S_15]~(12-)clusters arrange regularly along the diagonals of ab and ac planes to form a 3D tunnel structure with charge-balanced Na~+ cations located inside the tunnels. Note that SnS_4 tetrahedra connect with each other by sharing comers to form infinite _∞[SnS_3]_n? chains that are rarely found in known alkali-thiostannates_Remarkably,Na_2ZnSn_2S_6 exhibits the good NLO performances,such as large SHG response(4 times that of AgGaS_2 at55-88 um under the 2.09 um radiation), wide infrared(IR) transmission region(0.35-25 um) and relatively high laser damage threshold(77.5 MW/cm~2, ~2 times that of AgGaS_2). Theoretical calculations have also been performed to investigate the structure-property relationship and the results indicate that the origin of NLO effect can be attributed to the penta-tetrahedra[ZnSn_4]~(12-) clusters. A sodium zinc thiostannate Na_2ZnSn_2S_6 is reported with its interesting structural features and property characteristics. It crystallizes in the polar space% xou ^ Fddl (No.43) with the unit cell parameters of a = 12.945 (3) , b = 23.556 ) , c = 7.2774 (15) and Z = 8. In its structure, Zn-centered penta-tetrahedra [ZnSn_4S_15] ~ (12-) clusters arrange regularly along the diagonals of ab and ac planes to form a 3D tunnel structure Note that SnS_4 tetrahedra connect with each other by sharing comers to form infinite _∞ [SnS_3] _n? chains that are rarely found in known alkali-thiostannates_Remarkably, Na_2ZnSn_2S_6 exhibits the good NLO performances such as large SHG response (4 times that of AgGaS_2 at 55-88 um under the 2.09 um radiation), wide infrared (IR) transmission region (0.35-25 um) and relatively high laser damage threshold (77.5 MW / cm 2 , ~ 2 times that of AgGaS_2). Theoretical calculations have also been conducted to investigate t he structure-property relationship and the results indicate that the origin of NLO effect can be attributed to the penta-tetrahedra [ZnSn_4] ~ (12-) clusters.