Abstract: Accurate selection and extraction of specific wavelength optical signals are important demand in dense wavelength division multiplexing system. A novel optical filter which combining fiber ring resonator with mach-zehnder interferometer was proposed in this paper. Transfer function of this structure was deduced by the signal flow graph method and optical fiber resonant ring theory, and the analysis of simulation was present.The results show that based on the phase modification provided by the fiber ring resonator and optimizing the coupling coefficient in this structure, a flatness filtering response with square passband shape and high quality factor was 2.815×10³.Compared with other forms of fiber ring assist MZI filters, the novel fiber could provide better quality factor, which is helpful to select filtering frequency in dense wavelength division multiplexing system.