This paper presents a distributed optical fiber sensing system based on the state of polarization(SOP) changes delay estimation.Two optical signals are fed into the sensing fiber bi-directionally.The vibration will result in the polarization of light changes both forward and backward.At the end of each side,a polarizer is employed to convert polarization changes to light intensity information.The time delay between two signals indicates the location of the vibration.The analysis of the reason for the formation of two signals′ delay is performed,the correlation algorithm is used to calculate the time delay and the distributed sensing & positioning principles are clarified.Experiments are done to show the positioning accuracy of the principle,where good results are obtained from cross-correlation algorithm using FFT of the two-way electric signals,and are used to locate the position of the vibration.Results show that the positioning precision is up to 0.8%,and the maximum error is within 300 m.