Abstract: In order to reduce the influence of differential loss on the performance of four-frequency differential laser gyros, zero drift resulting from differential loss was analyzed theoretically according to semi-classical theory of gas lasers.Modes in the four-frequency differential laser gyro have different round trip loss due to different polarization,propagating direction.Light intensity and frequency of the lasing mode relate to the round trip loss,so zero drift will appear if influence of differential loss is not removed in the difference frequency.With self consistent equations,effects of detuning frequency,magnetic field,cavity length,reciprocal biasing,Faraday biasing,Doppler broadening and gas pressure on three coefficients of differential loss were analyzed.The results show that zero drift caused by differential polarization loss can be eliminated by application of magnetic field.However,zero drifts caused by differential direction loss and magnetic circular dichroism loss cannot be eliminated by varying magnetic field or detuning frequency.Coefficients of differential loss can be changed by changing gas pressure and neon isotope ratio.Therefore,besides reduction of differential loss themselves,optimization of gain medium will play a certain role in reducing zero drift caused by differential loss.