The effects of Pb, Ga doping on the crystal field, superexchange interaction and magneto-optical effect of Ce:YIG crystal are studied based on the quantum theory. It is found that when the molecular concentration of Ga doping is 12%, the molecular field on the Ce3+ ion is reduced by 51% and the difference of occupation probability between the two lowest energy levels of the Ce3+ is reduced by 49%. The Ga doping affects the crystal field and molecular field at the same time, while the Pb doping affects the crystal field only.