An all fiber Weak coherent optical fiber sensor(OFS) system for displacement measurement with a fiber Young′s decoding interferometer is proposed and demonstrated. The OFS of this kind avoids the fatale signal fading problem in whole coherence OFS, which is caused by the fluctuation of source wavelength and power, power loss in propagation, as well as the changes of polarization direction. The OFS system is not only of simple structure, easy adjustment, but also with high resolution up to 0.054 mm.