Abstract: To solve the zero-bias fluctuation problem of fiber-optic gyroscope (FOG) during its warm-up, the influences of initial alignment of 2π voltage and anterior gain fluctuation on the zerobias of FOG were analyzed, respectively. A novel hybrid modulation method, namely four-state modulation method was proposed, by which the drawback of conventional square wave modulation method can be overcome. Due to this proposed fourstate modulation and demodulation method, the rapid adjustment of Y waveguide 2π voltage was realized, which reduces the adjusting time of the warm-up from 5 minutes to 1 second. Owing to the analysis in the frequency domain, it was presented that the proposed four-state modulation and demodulation method can also suppress the crosstalk induced by modulation and suppress the influence of the anterior gain on the zerobias fluctuation. As a result, the change of the anterior gain fluctuation induced zero-bias of FOG during warmup is improved from 0.03(°)/h to 0.01(°)/h.