Two new mixed alkali metal lanthanum borates, LiCs2La(BO3)(2) and Li3K9La3(BO3)(7), were synthesized through a high-temperature solution method and millimeter-sized colorless crystals were obtained. The structures of LiCs2La(BO3)(2) and Li3K9La3(BO3)(7) have an intricate three-dimensional (3D) open framework with one-dimensional (1D) infinite channels surrounded by cation polyhedra and isolated BO3 units. In particular, LiCs2La(BO3)(2) exhibits a short cut-off edge <190 nm and a large experimental band gap of 6.20 eV, evidenced by diffuse reflection spectroscopy. Moreover, to better understand the relationship between the electronic structures and optical properties of LiCs2La(BO3)(2), theoretical calculations using density functional theory were performed. The result indicated that LiCs2La(BO3)(2) has a calculated birefringence of 0.036@1064 nm and the direct band gap is 4.54 eV. In addition, the effects of ion substitution on structural transition and the metal ion/boron ratio on dimensions of the B-O frameworks are also discussed in detail.