A sodium zinc thiostannate Na2ZnSn2S6 is reported with its interesting structural features and property characteristics. It crystallizes in the polar space group Fdd2 (No. 43) with the unit cell parameters of a = 12.945(3) angstrom, b = 23.556(5) angstrom, c = 7.2774(15) angstrom and Z = 8. In its structure, Zn-centered penta-tetrahedra [ZnSn4S15](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 SnS4 tetrahedra connect with each other by sharing corners to form infinite (infinity)[SnS3](n) chains that are rarely found in known alkali-thiostannates. Remarkably, Na2ZnSn2S6 exhibits the good NLO performances, such as large SHG response (4 times that of AgGaS2 at 55-88 mu m under the 2.09 mu m radiation), wide infrared (IR) transmission region (0.35-25 mu m) and relatively high laser damage threshold (77.5 MW/cm(2), similar to 2 times that of AgGaS2). 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 [ZnSn4S15](12-) clusters.