It is important for the strap down inertial navigation system to keep the zero-bias constant during the long working time of the RLG. The main factors affecting zero bias of the RLG are introdured in this paper. The mechanism of the zero-bias drift during the long working time of the RLG is analyzed. Experimental results show that when the laser gyro works continuously for one month，the zero-bias drift is about 0.0053°/h，and the positive counterclockwise beam loss difference changes by about 8%. The effect of loss difference on zero-bias of laser gyro is long and slow. In order to reduce the zero drift of laser gyro in long-term operation，the long-term stability of the loss difference of forward and retrograde waves should be improved.