A　linear　distributed　optical　fiber　sensor　based　on　the　principle　of　Sagnac　interferometer　is　developed　to　detect　pipeline　leakage　and　locate　leakage　point　in　real-time.　The　traditional　Sagnac　interferometer　is　converted　into　in-line　structure　by　using　a　Faraday　rotator　mirror,　and　the　in-line　structure　is　applied　easily　in　perplexing　pipelines.　The　representation　of　phase　change　caused　by　leakage　is　formulated　for　optic　signal.　The　measuring　principle　and　leaking　source　location　method　for　the　detection　system　are　analyzed.　An　experiment　is　designed　to　detect　leaking　source　and　locate　leaking　source,　and　the　leak　is　produced　at　4.020　km　from　Faraday　rotator　mirror.　The　results　verify　that　the　system　can　realize　leaking　source　detection　and　location　accurately　and　the　largest　location　error　is　less　than　118　m.