coupler is used to close the Sagnac loop to provide passive biasing of the interferometer, which allows a balanced-detection configuration in interferometric fiber-optic sensing. Yet, the system asynchrony caused by photodetectors and converters leads to the deviation of eventual retrieval result, which may affect the judgment of disturbance prediction. To improve the performance of fiber interferometers, a compensation demodulation method is proposed. Compared with previous compensation methods, which focus on a signal processing process, this work offers a solution through an approach in the signal source acquiring process, the origin of the problem. By inserting a controllable optic delay device, a modified interferometric fiber-optic structure is presented. Operation of the compensation demodulation method is verified in an experiment along a 30-km-long sensing fiber. The performance of the structure is also compared with the conventional one. Experimental results show that the improved structure is advantageous for considerate compensation effect, accurate calibration, and low location error for the application of disturbance localization.