Abstract: Resonator is the core sensing element of resonant optical gyroscope, its fineness is an important parameter of gyroscope. Fineness F directly affects the size of the sensitivity of fiber-optic gyroscope (FOG), so research on the characteristics of optical fiber ring resonator and its fineness is the key to optimize the design, manufacture and improve its performance. In this paper, the working principle of ring resonator was introduced in detail, including the coupling principle of the coupler structure and the transmission mode of light in resonator. Each influenced parameters of sensitivity and some parameters between coupling coefficient and loss were analyzed. Finally by considering the resonator of high precision and high Q, when the cavity size is 21.4 cm, the fineness can reach 170, the Q value can reach 3.34107, and at this point, the sensitivity is 0.48()/h. Moreover, the R-MOG closed loop experiment system was established and closed-loop response of a two-way rotation system was tested, which provides theoretical and experimental basis for the construction of gyro system.