A new alkali metals borate complex, Li2Na[B5O8(OH)2], has been successfully synthesized by a facile hydrothermal method. Single-crystal X-ray diffraction analysis reveals that it crystallizes in orthorhombic space group Pbcn with a=8.919(3)Å, b=9.181(3)Å, c=8.416(2)Å, Z=4. The crystal structure is constructed of two dimensional (2D) [(B5O8)(OH)2]∞ layers, while stacking along b axis and then connected by Li+ and Na+ cations to extend to 3D framework Li2Na[B5O8(OH)2]. UV-vis-NIR spectrum shows that Li2Na[B5O8(OH)2] possesses a wide range of transparency and a UV cut-off edge below 190 nm which indicates that it may be applied in the deep ultraviolet region. The calculated band structures and the density of states indicate that Li2Na[B5O8(OH)2] is a direct band gap compound with a band gap of 5.68 eV. In addition, IR spectroscopy, thermal stability and theoretical calculations of Li2Na[B5O8(OH)2] are also reported in this work.