For the modulation phenomenon of output light intensity in the laser gyro consisting of totally reflecting prisms during its dither bias, the effect of mechanical dither is calculated and analyzed systematically. By the numerical simulation and finite element analysis methods, an analytical expression for the output light intensity of the laser gyro is derived as a function of stress induced birefringence and deviation of photodetector. A new laser gyro type with the symmetic rectangle structure that can improve the stability of optical output is suggested for the first time. The results show that the position assembly accuracy of the photodetector and the stress induced birefringence have a significant effect on light intensity modulation. Reducing the photodetector deviation and using symmetic prisms material with a suitable refractive index, the extent of intensity modulation could be reduced by over 52.63%. The analytic study provides an important reference for improving the quality of output light and the reliability of prisms laser gyro.