The electronic structure and the anisotropic optical properties of lanthanum doped in KSr2Nb5O15 (KSN-La) were investigated by the first principles calculation dependent on the density functional theory, which was then supported by experiments. With La3+ doping in the KSN lattice, the band gap of KSN-La becomes smaller and the lattice structure transforms from tetragonal to orthorhombic due to the imbalance of charge of the KSN structure affected by the La3+ doping. The analysis of the anisotropic optical properties showed that the value of optical dielectric constant along [001] direction was larger compared to [010] and [100]. The calculated dielectric properties were in good accordance to the experimental results. KSN-La presented a larger dielectric constant, provided that La3+ was able to compensate the vacancies of the A sites of KSN, which were generated during the sintering process, leading to an effective enhancement of the dielectric constant.