Nonreciprocal structure open-loop fiber optic gyroscope(FOG)composed of 3×3 coupler has the advantages of high-sensitivity and wide-gauging range. The signal detection plays important role in the FOG system. The demodulation precision influences the maximum resolution of the gyro directly. According to characteristics of output signal model of FOG which uses 3×3 coupler,new signal demodulation algorithm is adopted. It can accurately demodulation arbitrary input of dynamic range. It overcomes disadvantage which is lowsensitivity round±π/2 work point,and improves precision of measured signal. Experimental results indicate that algorithm demodulate the measured physical variable quickly and exactly.