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Borate crystals are widely applied in solid‐state laser systems for their superior properties of wide transparency range, large nonlinear optical (NLO) coefficients, and high resistance to laser damage. Previous theoretical studies all reference the bandgaps to the UV absorption cut‐offs. Herein, through the calculated band structures of three widely applied borate NLO crystals, namely, β‐BaB2O4 (BBO), LiB3O5 (LBO), and KBe2BO3F2 (KBBF), it is found that the bandgaps are always much wider than the absorption cut‐offs by over at least 1 eV. Therefore, an excitonic mechanism is proposed for the band‐edge absorptions. From the detailed band structures of the frontier orbitals, with a Wannier−Mott type approximation to the excitons, and reduced screening from lowered effective dielectric constants, the binding energies of the excitons in the three mentioned borate crystals are obtained. It is found that the absorption cut‐offs agree with the transition energies of the excitons very well for all the three crystals. The implication of these findings helps to find ways of the absorption decrease and laser damage prevention of those crystals.