With the introduction of an alkali metal into the B-O framework, two new alkali metal borate crystals, Li5Cs2B2O14 and Li4CsB5O10, have been obtained for the first time. Both compounds obey a general formula of LimCsnBm+nO2(m+n) (m + n = 5, 7; m > n). The two crystals have different three-dimensional (3D) framework structures composed by LiOn (n= 4, 5), CsO10, BO3, and BO4 units. Li5Cs2B7O14 crystallizes into the polar and noncentrosymmetric space group Ama2, while Li4CsB5O10 belongs to the nonpolar and centrosymmetric space group P2(1)/c. Detailed structure comparison analysis indicates that the different arrangements of the anionic groups in LimCs7-mB7O14 (m = 4, 5) and LimCs5-mB5O10 (m = 3, 4) may arise from the cation size effects, bond-valence requirements, and differences of coordination environment. In addition, in order to get better understandings of electronic structures and linear optical properties, we also carried out first-principle theoretical studies.