A　Lock-in　Amplifier　based　on　LabVIEW　platform　is　developed　to　improve　the　signal　detection　level　of　optical　fiber　sensor　and　is　applied　to　demodulate　interference　signal　phase　from　optical　fiber　pipeline　leakage　detecting　system　based　on　Sagnac　interferometer.　The　demodulation　principle　of　Lock-in　Amplifier　and　the　method　of　leaking　source　location　for　the　detection　system　are　analyzed,　and　an　experiment　is　designed　to　detect　leaking　source　and　locate　leaking　source　when　the　leakage　occurs　at　5.0　km.　The　results　verify　that　the　developed　Lock-in　Amplifier　can　demodulate　leakage　signal,　and　the　leaking　point　can　be　located　accurately.