Resonant fiber optic gyro (RFOG) has attracted many attentions due to its advantages such as short-sensitive fiber and small size. The influences of laser linewidth, fiber ring resonator (FRR) and detection technology on the sensitivity of RFOG are analyzed. According to the detection method for RFOG signal, a formula of RFOG sensitivity limited by photoelectric detector sensitivity and a correction formula of RFOG sensitivity considering laser linewidth and 90° polarization-axis rotated splice are derived. The sensitivity influenced by error and noise of digital phase modulation voltage is studied based on multi-beam interference principle. The simulation results show that, in order to get high sensitivity, the length of FRR should be selected according to laser linewidth, and the length of two sections of fibers in both sides of 90° polarization-axis rotated splice should be set as odd times of the half-beat length of polarization maintaining fiber. Additionally, the root-mean-square of digital phase modulation voltage noise should be less than 0.22 V. The work provides theoretical guidance for the design of high precision RFOG.