The bias stability is an important parameter of Fiber Optic Gyro (FOG). The Y waveguide 2π voltage and the crosstalk, to some extent, decide the performance of the bias stability. Based on square wave modulation, a double-square wave modulation method is proposed, which can perform the function of rapid adjustment of Y waveguide 2π voltage through four-state modulation and demodulation. The adjusting time around zero rotation speed is reduced from 300s to 1s. Meanwhile, the coherent detection theory is applied to make a study of the crosstalk of the square wave modulation and the double-square wave modulation, which draws a conclusion that the double-square wave modulation can suppress crosstalk. Experimental results show that the crosstalk can be reduced from 0.22o/h to 0.04o/h, and the FOG’s bias stability can be improved.