The light path stability of ring laser gyro determines its performance, while the internal and external pressure difference of resonator is one of the important factors influencing the light path stability of gyro. To study the low air pressure assembly causing light path variations and the loss changes of ring laser gyro, the micro-deformation of mirrors was simulated and calculated by ANSYS software. The results indicate that under the condition of the low air pressure, there is maximum-deformation in the center of the mirror, therein to, the maximum-deformation of spherical mirror is 545 nm, and the maximum-deformation of plane mirror is 31 nm, thus this deformation leads to the light path′s deviation of 390 nm from the center of diaphragm and brings out a curvature variation of 174 mm for the spherical mirror. Furthermore, the deformation influence on the light path variation and the loss changes of ring laser gyro is calculated in theory,the results show that this deformation synthetically cause about 2%~3% loss variation of resonator, among them the variation of curvature is the main cause of loss variation. To increase the groove depth or decrease the air difference between outer and inner of the resonator will effectively reduce the light path variation. A test system is built and verifies that the results of calculation are correct.