The performance of the resonator integrated optic gyro (RIOG) is inevitably influenced by the intensity variation of the laser. In this work, the effect of intensity variation of the laser is mathematically formulized, analyzed, and experimentally validated, to our knowledge for the first time. First, the demodulated curves with different light intensities input of the integrated optical resonator (IOR) are simulated; the relationship between the slope of the demodulated curve near the resonant point and the light intensity input of the IOR is obtained. Second, the amplitudes of the output square waveforms with different zero biases are demonstrated, and it can be concluded that the effect of intensity variation has a high correlation with the nonzero bias between the clockwise and counterclockwise resonant frequency. Third, the experimental setup is constructed and the related measurements are performed, the test results are in good agreement with the analytical and numerical simulation, and in order to reach the limited ultimate sensitivity of the RIOG, it is necessary to restrict the nonreciprocal zero bias within 8.1 deg/s under an open-loop output scheme. Furthermore, to eliminate the noise induced by intensity variation of the laser and realize a high performance RIOG, a closed-loop operation is required.