A fiber-optic distributed sensor based on Michelson interferometers for detecting the time-varying disturbance was presented in the paper. The proposed sensor is constituted of two Michelson interferometers with an optical fiber delay loop. The disturbance causes a phase modulation which is detected by the proposed sensor and converted to the location information. The interference signals received by the photo detectors through capacitive coupling separated directly to eliminate the DC component are preprocessed by the peak-to-peak value calculation method. The phase information of the preprocessed signals is acquired by the Hilbert transform, phase unwrapping and the corresponding trigonometric equations calculation. The frequency spectrum analysis and the mathematical operations are then conducted to locate the disturbance. Experimental results obtained with a 20 km long sensing fiber are discussed in the paper. The proposed distributed disturbance sensor has the significant advantages of capability of real time sensing, insensitive to the polarization state and low cost.