Resonator fiber optic gyro(RFOG) is a high accuracy inertial rotation sensor based on Sagnac effect. As one of reciprocity noises in RFOG, input intensity fluctuation of fiber ring resonator(FRR) would lead to detection error. Firstly, the mechanism of noise induced by FRR input intensity fluctuation was investigated. Transmission characters of FRR and demodulation output with different input intensities were researched theoretically and experimentally. The expression of detection error caused by input intensity fluctuation was derived. When rotation rate was 500 (°)/s and input intensity was 0.69 mW, a variation of 0.007 5 mW would cause an error as large as 5.26 (°)/s. Secondly, the effect of input intensity fluctuation on scale factor was studied. It was found that the linear region of demodulation curve would be distorted as input intensity fluctuation increase and scale factor nonlinearity of gyro output would deteriorate too. This work proved a reference for estimating input intensity fluctuation noise in a RFOG system.