介绍了交变法拉第效应的磁光调制测量原理和方法，分析了亚铁磁性材料法拉第Ｆｒａｄａｙ效应和磁光调制深度与交变磁场频率间的关系，证明了交变法拉第旋转幅值θ０的实部θ＇０小于静磁场下的θ＇ｓ，而虚部θ″０大于θ″ｓ，理论和实验结果相吻合。由此得出结论，采用具有磁圆二向色性的磁光材料做成的调制器不可能达到彻底的开关状态。 The principle and method of magneto-optical modulation measurement of alternating Faraday effect are introduced. The relationship between Faraday’s Fraday effect and magneto-optical modulation depth of ferrimagnetic material and frequency of alternating magnetic field is analyzed, and the real part of the alternating Faraday rotation amplitude θ0 θ’0 is smaller than θ’s in static magnetic field, and the imaginary part θ “0 is greater than θ” s. The theoretical and experimental results are in good agreement. This concludes that a modulator made of a magneto-optical material having a circular dichroism can not achieve a complete switching state.