Abstract: The mirror transducer assembly causing light path variation of ring laser gyro is analyzed. This analysis reveals how the action mechanism of the mirror transducer assembly influences light path in ring laser gyro and to what extent it may have. The calculated results indicate that mirror tilt can induce a light path shift of about 10～20％ aperture diameter at the aperture transverse section. A test system is built to verify that it is the tilt of mirror transducer assembly which causes the light path variation at the aperture. At last, the impact of the action mechanism of light path variation on ring laser gyro performance is discussed.