Back-reflection is one of the important noise sources in the resonator optic gyro (ROG). Using optical field overlapping method, the single-reflection-model inside the ring resonator was set up and the relation between the scale-factor error and the parameters about reflection noise was gotten. By the numerical analysis in Matlab, the back-reflection noise in the resonator fiber-optic gyro (R-FOG) and the resonator micro-optic gyro (R-MOG) was analyzed and compared in theory. The open-loop output signal in ROG was detected by taking some countermeasure to overcome the noise caused by the backscattering. It was proved theoretically and experimentally that the noise caused by the back-reflection influences the R-MOG more severely.