In order to improve the transmission performance of the conventional Mach-Zehnder interferometer (MZI), a novel interleaver combining ring resonator with a single fiber coupler of 3 dB direction is proposed. The ring resonator shaped like an “8” form is based on double-coupler and single-mode fiber. Based on the phase modification provided by the ring resonator, a flatness filtering response is obtained by optimizing the coupling angle of resonator. Compared with the traditional MZI-interleaver and two-stage MZI-interleaver, the isolation in stopband and the rolloff in transition band are strengthened. Compared with the interleaver based on an asymmetrical MZI with a resonator in one arm, there is no difference between the intensities of two coherent light beams in the condition of considering the influence of the propagation loss. Theoretical analysis shows that the influence of the propagation loss on extinction ratio can be effectively reduced by proposed interleaver.