A novel structure of high extinction ratio comb filter based on dual-pass Mach-Zehnder interferometer is proposed. The comb filter is structured by adding an optical isolator to the two outputs of the conventional single-pass Mach-Zehnder interferometer. The characteristic of the transmission spectrum is studied theoretically and experimentally. The numerical results show that the extinction ratio can be enhanced greatly in comparison with the conventional single-pass Mach-Zehnder interferometer in the same parameters. The improvement of the extinction ratio mainly depends on the difference between the two arms of the Mach-Zehnder interferometer. When the difference is 0.7366 mm, the extinction ratio is improved 20 dB. The experimental results show that the extinction ratio of the dual-pass Mach-Zehnder interferometer is 25.8 dB,the extinction ratio is improvedly 13 dB.