The signal detection technique to control the working-point by dosed-loop is an effective method of interfemmetric fiber-optic sensors. The arithmetic of signal demodlation is improved based on the traditional method,which achieves the wider dynamic range,the bigger speed of detection and the higher precision of demodulation. The demodulation error is analyzed and simulated theoretically. The conclusion is that the measurable amplitude of the dynamic phase change and the permitted varying range of the working-point restrict each other. To validate the improved arithmetic, a simple Macb-Zehnder interferometer is designed. The experimental results have proved its correctness.