The electronic structure of Ce:YIG is calculated by using DV Xα (discrete variational Xα)method. The results indicate that the couple orbits among 5d, 4f, and 3d orbits are existent because of the substitution of Ce 3+ ion. The couple orbits have large spin orbit interaction. The results also show that there are electronic transitions Fe 3+ (3d) →Ce 3+ (4f) that may be the origin of transition centers of 1.5 eV and 2.1 eV shown the large optical absorption. The density of transition center is proportional to cerium content. The optical absorption spectra of Y 3-x Ce x Fe 5O 8 on x=0, 0.3, 0.7, 1.0, 2.0 are calculated between 1.0 eV～3.2 eV. The calculated results are consistent with experimental data.