Since the asymmetry of laser gain leads to the unequal of scale factor corrections between left circularly polarized and right circularly polarized gyros in the cavity, zero-lock laser gyro (ZLG) bias is sensitive to magnetic field when frequency stabilization point is not matched with magnetic insensitive point, which degrades the precision and limits work condition of ZLG. The relation between scale factor corrections and gain-to-loss ratio is analyzed according to Lamb semiclassical theory, as well as the relation between magnetic insensitive point and discharge current. The influence of discharge current changing on control signal is eliminated from signal processing system and experimental verification is performed. It is proved by both theoretical calculation and experimental results that the magnetic insensitive point varies linearly with gain-to-loss ratio as the discharge current changes linearly, which means that the magnetic sensitivity can be reduced by changing discharge current.