A method for designing optic fiber ring in Brillouin Fiber Optic Gyro (B-FOG) is presented. Based on the principle of Brillouin fiber optic gyroscope,the optical Jones matrix theory and stimulated Brillouin scattering classical theory are utilized to describe the characteristic of Brillouin optical resonator in detail. Through meticulous analysis of light intensity inside and outside the resonator,the requirement of coupling state and pump light intensity which the fiber ring resonator in B-FOG needs is obtained. In the end,the reasonable Brillouin fiber ring resonator in B-FOG is designed,and test results show that when this method is used to set B-FOG working point,the greatest transmission efficiency of the Stokes light is ensured and the granular noise is suppressed.