In recent years, hydroxyborates with excellent properties have attracted much attention. Through dedicated efforts, three new hydroxyborates-K2B5O8(OH), CsB5O6(OH)4, and CsB5O7(OH)2-have been successfully synthesized in a closed system. The ultraviolet (UV) cut-off edges of both K2B5O8(OH) and CsB5O7(OH)2 are below 200 nm, indicating their potential as candidates for deep-ultraviolet (DUV) materials. Furthermore, K2B5O8(OH) exhibits nonlinear optical (NLO) activity and demonstrates significant second harmonic generation (SHG) effects, approximately 2 × KH2PO4 (KDP). Interestingly, although all three compounds are alkali metal borates containing five boron atoms, the calculated birefringence is 0.025 at 1064 nm for K2B5O8(OH), whereas it is 0.067 and 0.070 at 1064 nm for CsB5O6(OH)4 and CsB5O7(OH)2, respectively, which are about three times that of K2B5O8(OH). The reason for the nearly threefold difference in birefringence is analyzed from the view of the structure-property relationship. Furthermore, the effect of the number of hydroxyl groups and shared oxygen atoms on the structural dimensions, birefringence, and band gaps in all alkali and alkaline-earth metal hydroxyborates with five boron atoms has been studied and analyzed. A solid foundation for the use of hydroxyl groups to tune and design structures has been provided.