The refractive indices and quadratic electro-optic effect in terms of the coefficients (R11-R12) in a paraelectric K0.95Na0.05Ta0.58Nb0.42O3 single crystal were measured. The dispersion of the refractive index was described exactly by a single-term Sellmeier equation. We found an obvious dispersion of the electro-optic coefficients (R11-R12), and the coefficients decreased quickly with increasing wavelength above the Curie temperature. Following [J. Appl. Phys.40, 720 (1969)], we obtained a dispersion equation for the electro-optic effect in a paraelectric potassium sodium tantalate niobate single crystal. The experimental results agreed well with the dispersion model.