In order to separate and extract the Brillouin scattering signal from backscattering light in BOTDR distributed optical fiber sensing technology,a high extinction ratio dual-pass tunable Mach-Zehnder interferometer was designed.The interferometer consists of two 3dB coupler,electric fiber delay line,polarization controller and optical isolator.The performance of the M-Z interferometer was tested by using a C band broadband light source (ASE).Pulse light which pulse wide is 100 ns,repetition frequency is 20 kHz was injected into a single mode fiber with the length of 5 km.The backscattering light that generated from the fiber was filtered through the M-Z interferometer,then the output spectrum signal was detected by the spectrometer.Experimental results show that the M-Z interferometer can achieve a large range of high precision adjustable filtering function,and Rayleigh scattering light was suppressed greater than 20 dB.It can effectively separate and extract the Brillouin scattering light signals from the backscattering light.