Transparent ceramics have been widely researched for their broad range of applications, e.g. from optical windows to laser and optoelectronic switches. However, the challenge is to obtain the optical materials with high refractive index to miniaturize optical functional elements, such as lens for optical information storage and waveguides for flat optical components. The hexagonal complex perovskite Ba(Mg1/3Nb2/3)O3(BMN) ceramic, being widely researched as a type of microwave dielectric ceramics, presents the excellent dielectric properties such as high dielectric constant and high Q value, which indicate its potential application as optical materials. In this paper, the electronic structure of BMN is calculated by using the first principle method, to analyze and predict its intrinsic optical properties. The hexagonal complex perovskite BMN ceramic is synthesized using conventional solid-state reaction at 1600 ℃ for 24 h. The structure parameters are obtained through Rietveld refinement of X-ray diffraction data. The crystal model is established, based on the Rietveld refinement result of the XRD test on synthesized BMN (with the weighted profile R-factor Rwp=6.73%, the profile R-factor Rp=5.05%), and then the crystal geometry optimized. With the optimized crystal model, the energy band structure, density of states and optical properties of BMN are calculated using the first principle method based on density functional theory (DFT) with local density approximation (LDA). Results show that BMN has an indirect band gap of 2.728 eV. There are the strong ionic interactions between Mg and O as well as Ba and O, while there is covalent interaction between Nb and O. The energy band near the Fermi level is mainly occupied by O-2p and Nb-4d electrons, which forms the d-p hybrid orbits. With real band gap correction, the optical properties of BMN are obtained from the definition of direct transition probability and the Kramers-Kronig dispersion relations along the polarization directions 100 and 001, including the complex dielectric function, absorption coefficients and reflectivity, respectively. It is shown that the optical properties of BMN are nearly isotropic. According to the Lambert-Beer's law, the intrinsic transmittance of BMN ranges from 77% to 83% in the visible region, and its refractive index is dispersive, ranging from 1.91 to 2.14. Experimental test results are consistent with the theoretical calculation results.