Bias stability is an important performance of gyro. But in digital closed loop fiber optical gyro(FOG), it is influenced extremely by the electrical crosstalk in the electronics. Output drift can be reduced by using random modulation.The crosstalk of square wave modulation and random modulation was analyzed through fast Fourier transformation(FFT). When square wave modulation is used, the crosstalk result has the same square wave frequency with the photodetector analog output. So it can not be filtered in the gyro electronic and bias error arises in output. Further more , it causes the output dependence on the light intensity. When random modulation is used, the crosstalk result has different frequency with the photodetector analog output, so the output of FOG has more stablity. The random modulation point is -3π/2,-π/2,π/2,3π/2, which is the most sensitive point. The modulation wave has even harmonious of square wave frequency. Experimental results indicate that bias error can be restrained and the bias stability of output increased when random modulation is used in digital closed loop FOG.