The　third　damage　action,　human　disruption　behavior,　is　one　of　the　major　threats　to　pipeline　operation.　It　is　necessary　to　monitor　and　locate　the　perturbation　behavior　that　may　threat　pipeline　safety　in　real-time.　Therefore,　a　new　pipeline　security　monitoring　system　is　designed　by　using　line-structure　Sagnac　optic　fiber　interferemeter　with　the　characteristic　of　3×3　coupler　that　can　modulate　the　phase　of　optic　signal,　with　no　need　for　phase　modulation　and　demodulation.　The　optic　structure　of　the　new　system　is　simplified,　the　signal　processing　accuracy　is　improved,　and　the　polarization　effect　is　reduced.　The　working　principle　of　the　monitoring　in　ideal　condition　is　described,　the　phase　demodulation　is　analyzed　and　the　location　of　the　possible　damage　action　point　is　formulated.　By　using　simulink,　the　optic　signal　propagation　and　interference　is　simulated　and　interference　signals　in　different　frequencies　are　obtained.　The　results　validate　the　feasibility　of　the　monitoring　system　and　indicate　that　the　low　frequency　signal　less　than　1　kHz　resulting　from　the　human　damage　action　can　be　detected.　The　disturbance　set　at　10　km　can　be　located　by　calculating　the　demodulation　signal　accurately　over　a　long　monitoring　distance.　©,　2015,　Nanjing　University　of　Aeronautics　an　Astronautics.　All　right　reserved.