The correlation between the odd and even time slot outputs of the fiber optic gyro with double optical length is used for the correlation-cancellation processing of gyro zero offset error. To verify the method, the fiber optic gyro with double optical length is at room temperature, with moving average time of 100 s, the per second output biases of odd and even time slot are -0.1°/h and 0.08°/h, after processing of correlation cancellation, the bias is modified and the bias stability is increased from 0.008°/h to 0.002°/h. The correlation counteraction signal processing method is fast, simple and effective, which provides a new idea for the precision optimization of the fiber optic gyro with double optical length.