A series of new rare-earth borate crystals K7MRE2B15O30 (M = Zn, Cd, Pb; RE = Sc, Y, Gd, Lu) were synthesized by solid-state reaction method. All of the title compounds crystallize in the noncentrosymmetric trigonal space group R32, and their structures consist of the B5O10 groups and the KO8, KO6, MO6, and REO6 polyhedra. Each isolated B5O10 group is connected by the REO6 (Sc3+, Y3+, Ge, Lu3+) groups forming an intricate three-dimensional network, and the K+ and M2+ (Zn2+, Cd2+, Pb2+) cations fill into the void space. Because of the introduction of different nonlinear optical (NLO)-active structural units (cations with d(10) electronic configuration and stereoactive lone pairs) into rare-earth metal borates, these compounds exhibit second harmonic generation (SHG) responses ranging from 1.5 to 2.1 times that of KH2PO4 (KDP), and are phase-matchable. Interestingly, this work indicates that the NLO properties can be artificially adjusted by systematically replacing the bivalent M2+ and trivalent RE3+ cations with NLO-active structural units, which provides a new way to design new NLO materials. Thermal analyses, IR spectra, and UV-Vis-NIR diffuse reflectance spectra were also reported in this work. In addition, the first-principles calculation was employed for better understanding of the structure-properties relationships of these compounds. Especially, the origins of SHG responses were well demonstrated by the SHG-density technique.