In order to improve the precision of Nonplanar Four Mode Differential Laser Gyros (NFMDLG), a compensation method by using reciprocal biasing is proposed. Expressions of reciprocal biasing and bias of the NFMDLG are derived according to the classic theory of a gas laser. The bias characteristics of the left circularly polarized gyro in the NFMDLG on the left and right sides of a gain curve are analyzed for two cases, respectively. Then,it points out that the bias caused by a magnetic circular dichroism loss is identical in number while reverse in sign for the two cases. An experimental system is established to measure the reciprocal biasing and bias synchronously,which verifies the theoretical analysis by varying the discharge current and temperature. Finally,a method is proposed to compensate the bias error of NFMDLG with reciprocal biasing. Experiments show that the bias drift resulted from temperature variation in the range of 10-60 ℃ is less than 0.01 Hz,and that resulted from 0.2 mA current variation is reduced by four-fifths after compensation. In conclusion,the reciprocal biasing can compensate effectively the bias caused by the mode pulling effect.