Pure perovskite phase and crack-free KTa_0.5Nb_0.5O₃ thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. The structure and orientation were analyzed by X-ray diffraction. The optical properties were investigated by an ellipsometer. The relationship between the refractive index dispersive behavior and internal structure was analyzed by Sellmeier dispersion model and single electronic oscillator approximation. The parameters of room temperature monomial Sellmeier oscillator were calculated. And the refractive index dispersive parameter E₀/S₀ of KTa_0.5Nb_0.5O₃ thin films on Pt/Ti/SiO₂/Si substrates is (6.72 ± 0.04) × 10⁻¹⁴ eV m², which is consistent with those of KTN crystals and compounds with ABO₃ perovskite type structure.