In order to improve the transmission performance of the conventional Mach-Zehnder (MZ) interferometer, a novel interleaver based on MZ interferometer with a double-coupler and single mode fiber resonator in one arm is presented. The output expression is derived and numerical simulation is performed. The simulation indicates that the bandwidth of 0.5 dB passband and 25 dB stopband of proposed interleaver are simultaneously improved remarkably, which are much wider than those of the conventional MZ interferometer. The filtering performance of this proposed interleaver, which achieves a nearly square spectrum response, is much better than that of the conventional MZ. Influence on its transimission characteristics by the parameters caused by double-coupler resonator, coupling ratio and length of resonator, is also discussed, and the variations ranges of the parameters are also given for the communication requirements.