Abstract: To improve the filtering performance of conventional Mach-Zehnder interferometer (MZI) filter, a novel structure which combining ring resonator with MZI is proposed in this paper. Transfer function of the improvement filter was established by using the Signal Flow Graph and the characteristic of the output spectrum was simulated as well. The results of numerical simulation indicate that when MZI with the length difference of interference arms is chosen, through the phase modification provided by the ring resonator and the optimization of the coupling coefficient, the spectrum of output port has the properties of narrow band with high extinction ratio. When the MZI with the length identical of interference arms is adopted, by optimizing the parameters of the structure, a maximum flatness filtering response is obtained, the isolation in stopband(>40dB) and the rolloff in transition band are strengthened. Compared with the filtering performance of conventional MZI filter, the novel optical filter proposed in this paper improved the performance obviously.