It is important for the laser technology and related scientific fields to develop nonlinear optical(NLO) crystals. The development of novel NLO crystals with superior performance requires the understanding of the structure-property relationship. In this paper, the first-principles studies based on the density functional theory and their applications to elucidate the microscopic origins for the linear and nonlinear optical properties of NLO borates were reviewed. The ab initio approaches have a capability to accurately predict the optical properties of NLO borates, and the developed analysis tools can be applied to investigate their intrinsic mechanism. It is anticipated that the first-principles approaches could be used to select the effective NLO materials It is important for the laser technology and related scientific fields to develop nonlinear optical (NLO) crystals. The development of novel NLO crystals with superior performance requires the understanding of the structure-property relationship. In this paper, the first-principles studies based on the density functional theory and their applications to elucidate the microscopic origins for the linear and nonlinear optical properties of NLO borates were reviewed. The ab initio approaches have a capability to accurately predict the optical properties of NLO borates, and the developed analysis tools can be applied to investigate their intrinsic mechanism. It is expected that the first-principles approaches could be used to select the effective NLO materials