Compared to the dithered ring laser gyro with the linearly polarized light running in the cavity, the Zerolock Laser Gyro(ZLG) with the circularly polarized light running in the cavity is more sensitive to the magnetic field. In order to reduce the magnetic sensitivity of the ZLG, the magnetic sensitivity of the ZLG is theoretically analyzed. When the relative sacle factor corrections of left and right circularly polarized gyros are equal, the bias of the ZLG will not vary with the magnetic field. Further more, the linearity of relative scale factor correction will be improved. The magnetic sensitivity of the ZLG is experimentally tested, and the results indicate that the insensitivity point with respect to the magnetic field is existent, which is consistant with the theroretical analysis. Compared to the method of traditional passive magnetic shielding, the method of operating ZLG on the insensitive point by active control is proved to be of great practical value for reducing the magnetic sensitivity and improving the accuracy of the ZLG.