There are many reflected and refracted points in prism ring cavity. The angle of work plane, pyramidal error and cavity adjustment are strictly required in order to ensure the stability of gyroscopes. We set the angle error of prisms and sphere eccentricity as the perturbation terms in meridian and sagittal planes, and amend the transmission matrices of light reflecting and refracting in prism surfaces. We establish physical models of light geometric deflection loss and light path non-coplanar loss in prism ring cavity, and analyze the rate of deviation from the design position by the revised transmission matrices. Based on the assorted screen and resonant loss controlled, we design a new way for cavity adjustment. The experimental results show that the new cavity adjustment method makes once pass rate rise to above 90% from 75%.