A phase-sensitive optical-time-domain-reflectometer (φ-OTDR) using a fiber laser with high power and narrow linewidth is described. The sensing element is a single-mode telecommunication fiber cable with a 3 mm diameter buried outdoors. The phase changes of Rayleigh scattering light resulting from the pressure (vibration) of the intruder on the ground immediately above or near the buried fiber. By subtracting a present φ-OTDR trace signal from an earlier one with a time difference of 0.1 s, the system can locate the intrusion point where the optical intensity will change for the reason of interference. By processing the returned signal properly, a distance range of 14 km can be reached, with the best spatial resolution of ～50 m reported to date, and a signal-to-noise ratio of ～12 dB, to the best of our knowledge. Such a distributed sensing system is anticipated to be used widely in safety monitoring of military bases, national boarders, nuclear facilities, electrical power generation stations, jails, et al..