In order to elucidate the mutual influence mechanism among the factors in the magneto-optic modulation process based on Faraday effect, a multi-field coupling theory is introduced. A transmission model of polarized light is constructed at the condition of magneto-optic coupling when we comprehensively consider the change of internal parameters of the material structure under the action of the magnetic field and combine the Maxwell′s equation, and the final expression of the Faraday rotation angle is obtained. Coupled simulating and numerical solving are carried out with multi-field coupling software based on finite element method. The simulation results show that, a deflection angle of 2.245° occurs when the polarized light with a wavelength of 1064 nm passes through a 3 cm Tb3Ga5O12 (TGG) magneto-optic glass under magnetic induction intensity of 0.0373 T in a 10 A current excitation solenoid, while a deflection angle of 2.023° can be obtained by MR3-2 magneto-optic glass under the same condition. The correctness of the magneto-optic coupling mechanism model is verified by the comparison between the simulation results and the theoretical calculation results.