This paper describes a novel signal-locating scheme for the ultra-long distance distributed optical phase sensing system.The scheme was realized by combining the double Mach-Zehnder(MZ) configuration and the correlation algorithm,which solves the locating problem of the detected signal in the sensing system.The studies show that the scheme can locate the signal in the range of 50 lm successfully.The minimum locating relative error of 0.19% is achieved when the sensing system is buried in the 0.5...