The first rubidium barium borate, RbBaB7O12, has been prepared. Through analysis of single crystal structures, RbBaB7O12 is built from a three-dimensional [B7O12](infinity) framework with two types of channels which are occupied by Rb+ and Ba2+ ions. The connection style of the fundamental building block (FBB) B7O15 is different from that of B-O FBBs existing in the other borates, which can be determined as the unprecedented unit. The structural performance relationship can be better understood by combining first-principles calculations and experimental results. For the RbBaB7O12, the indirect energy gap is 5.96 eV, which matches the experimental data (wavelength absorption < 190 nm). IR spectroscopy and thermal analysis have also been characterized. What's more, for all the available anhydrous hepta-borates, the structure comparisons about FBBs were carried out.