The tunability of the terahertz dielectric spectra of Y3Fe5O12(YIG) ceramics under external optical field were investigated at room temperature by using terahertz time-domain spectroscopy. The Y3Fe5O12 ceramics annealed at 1 100 and 1 300 ℃ were successfully synthesized by a modified Pechini process. The samples were characterized by X-ray diffraction and no impurity phase has been detected. The crystallite sizes of the YIG ceramics are calculated to be about 50 nm for TS=1 100 ℃ and 77 nm for TS=1 300 ℃, respectively. And Scanning Electron Microscope(SEM) images show that the grain size is positively correlated with the sintering temperature. Application of the optical field leads to an appreciable tuning of the dielectric constant of the sample sintered at 1 100 ℃, which reaches up to 17.3% at 0.6 THz. Meanwhile the dielectric loss tangent of the sample sintered at 1 100 ℃ changes about 123.9%. On the contrast, both the dielectric constant and the loss tangent for the sample sintered at 1 300 ℃ keep invariable with the light excitation. These findings are attributed to space charges hopping among the grain boundaries excited by the external optical field.