Three new complex borate compounds K7CaBi2B15O30, K7CaLa2B15O30 and K7BaBi2B15O30 have been synthesized by the high-temperature solution method. K7CaLa2B15O30 and K7CaBi2B15O30 crystallize in the chiral trigonal space group R32, while K7BaBi2B15O30 crystallizes in the noncentrosymmetric orthorhombic polar space group Pca2(1). All of the title compounds have similar three-dimensional crystal structures, which are composed of isolated B5O10 groups and LaO6 or BiO6 octahedra, and K+, Ca2+, and Ba2+ cations fill into the cavities to keep charge balance. Based on our research, in the system of K7MII RE2B15O30 (M-II = Ca, Sr, Ba, Zn, Cd, Pb, K/RE0.5; RE = Sc, Y, La, Gd, Lu, Bi), K7BaBi2B15O30 is unique and crystallizes in a different space group, which enriches the structural chemistry of borate. Detailed structural analyses indicate that the structural variation is due to the difference in size and coordination number of the alkaline-earth metal cations. Besides, UV-Vis-NIR spectroscopy analysis and the second-harmonic generation (SHG) measurement on the powder samples show that K7BaBi2B15O30 exhibits a UV cutoff edge (about 282 nm) and a moderate SHG response (about 0.6 x KDP). In addition, thermal analysis and infrared spectroscopy were also presented. To better understand the structure-property relationships of the title compounds, the first-principles calculations have been performed.