The optical system has been established in which the changes of polarization state for magneto-optic crystal can be measured. In this optical system, WGY model semiconductor laser is applied as the light source, and the tuning range of the intensity of magnetic field is 0～1500 mT. Depolarization effect in GdYBiIG crystal which often be used in optical isolator is measured in large variation range of adjustable magnetic field. The experiment result shows that polarization performance will be best in GdYBiIG crystal when it reaches saturated magnetization. And the depolarization effect occurs when the intensity of magnetic field increases over saturation intensity. The principle for magnetic depolarization effect is also given according to the experimental result. It presents that magnetic circle dichroism and magnetic linear birefringence are the reasons which cause depolarization effect. From these results, it is optimum to make sure that the magnetic field should be lager only a little than the saturation intensity of magnetic field.