The electronic structures and optical properties of both the perfect CsI crystal and the crystal containing a pair of V^1-_Cs-V¹⁺_I are calculated using CASTEP code with the lattice structure optimized. The calculated results indicate that the optical symmetry of the CsI crystal coincides with the lattice structure geometry of the CsI crystal. The absorption spectrum of the CsI crystal containing a pair of V^1-_Cs-V¹⁺_I also does not occur in the visible and near-ultraviolet range. It reveals that the existence of the pair of V^1-_Cs-V¹⁺_I in CsI crystal has no visible effects on the optical properties of the CsI crystal.