Based on the principle of molecular design and self-assembly, a novel rare earth based arsenotungstate was obtained by introducing rare earth LaⅢ ions into the lacunary polyoxotungstates clusters with vacancy structure by one-pot method with Na2WO4·2H2O, NaAsO2 and LaCl3·6H2O as the main materials. The structure and composition of this complex was confirmed by single-crystal X-ray diffraction, X-ray powder diffraction (PXRD), infrared spectroscopy (IR) and so on. The single crystal X-ray diffraction results show that the complex is in the monoclinic system, C2/c space group, and with the cell parameters of a=3.595 56(14) nm, b=1.222 48(4) nm, c=2.247 34(9) nm, β = 117.117 0(10)°, V=8.792 3(6) nm3, Z=4. The molecular formula of this compound is ［(CH3)2NH2］8H3.56［{LaO(H2O)2}{WO(H2O)}{W0.24(H2O)2}{AsW9O33}2］. And the polyoxoanion ［{LaO(H2O)2}{WO(H2O)}{W0.24(H2O)2}{AsW9O33}2］11.56- shows a novel sandwich type structure, which can be simply described as two trilacunary Keggin type{AsW9O33} units are linked by a {LaO(H2O)2} segment, a {WO(H2O)} segment and a {W0.24(H2O)2} segment. Catalytic degradation property of this compound for Rhodamine-B was further inverstigated. The results demonstrate that the compound shows good photocatalytic activity under the condition of 300 W Hg lamp irradiation, and the degradation efficiency reaches to 74%.