A　distributed　optical　fiber　sensor　system　is　constructed　based　on　the　principle　of　Sagnac　interferometer　with　3.　3　coupler　for　long-distance　pipeline　security　monitoring.　By　using　this　system,　disturbance　such　as　the　action　threat　to　the　security　of　pipeline　can　be　identified　and　be　located　in　real　time.　The　system　using　ring　structure　needs　no　phase　modulation　and　demodulation　because　the　3×3　coupler　can　modulate　the　phase　of　optic　signal;　then　the　precision　of　signal　processing　is　improved.　The　principle　of　the　monitoring　in　ideal　condition　is　put　forward,　and　the　phase　demodulation　and　the　location　of　the　possible　damage　action　point　are　analyzed　and　formulated.　The　optic　signal　propagation　and　interference　are　simulated,　and　interference　signals　in　different　frequencies　are　obtained.　The　results　validate　the　feasibility　of　the　system,　indicate　that　the　system　can　detect　the　low　frequency　signal　resulting　from　the　damage　action　and　locate　the　action　point　accurately　over　a　long　distance.　©　2015　Taylor　&　Francis　Group,　London.