The structure and the optical properties of the tridymite green color phosphor BaMgSiO4 :Eu are systematically examined. Due to the differing ion radii of Eu3+, Mg2+, Si4+, and the three different Ba 2+ sites in the crystal lattice, dopants can only replace the Ba2+ sites because the other sites are insufficiently large to hold the massive Eu3+. BaMgSiO4 is a promising candidate for inorganic photochromatic materials with excellent optical properties. It has attracted much attention in the world due to its applications in high‐density optical memory and light‐emitting diodes. Herein, the generalized gradient approximation and generalized gradient approximation plus Hubbard potential (GGA + U) are used to investigate the electronic structure and the entire optical properties of the parent and Eu‐doped BaMgSiO4, like dielectric function, refractive and extinction coefficients, the optical conductivity, reflectivity, and absorption spectra. It is found that the substitution of Eu ions reduces the energy bandgap of BaMgSiO4 and introduces the sub‐bands in between the valence and conduction bands. Findings show that the bandgap and optical characteristics of the BaMgSiO4 compound are described properly by the GGA + U method.