A new 3D extended heteropoly blue Ba4[SiW(V)4W(VI)8O40]·H2O () composed of twelve-coordinated α-Keggin anions [SiW(V)4W(VI)8O40](8-) and eight-coordinated Ba sites {BaO8} has been hydrothermally synthesized and fully characterized by elemental analysis, IR spectroscopy, XPS, single crystal X-ray and X-ray powder (XRPD) diffraction. represents the first inorganic 3D framework constructed from four-electron reduced α-Keggin anions linked by alkaline earth metals. The photocatalytic activity of has been evaluated for rhodamine B (RhB) degradation. exhibits excellent catalytic activity for the degradation of RhB in the presence of H2O2and the involvement of visible light makes a more complete degradation. The results of the current study suggest that multi-electron reduced polyoxometalates can catalyze efficient degradation of an organic dye with H2O2.