The dielectric properties of 0.55SrTiO₃-0.45NdAlO₃ ceramics under external optical fields were investigated by terahertz time-domain spectroscopy at room temperature. From the experimental results, it could be found that the tunability of permittivity with the external optical pump was reached up to 16% at 0.6 THz. And the change of refractive index has a linear relationship on scale with the applied external light power. These results could be explained by a built-in electric field caused by the excited free carriers in the ceramics.