Abstract: 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.