An all-fiber interleaver of Cascaded M-Z Interferometer (CMZI) composed of two 2×2 and two 3×3 fiber couplers with almost rectangular wavelength response was developed by using a new configuration. Based on the principle of characteristics of fiber transmissivity, the complex amplitude output spectra were established and described by the expression using the matrix transfer function. The analytical results indicate that when the third length difference of interference arms is as twice as the first length difference of interference arms, the coupling coefficient of the couplers is some certain values, and then a uniform flattop passband and similarity to square spectral response can be obtained by the configuration. The passband and stopband of optical interleaver are improved remarkably and the interval of peak output power is 1.6 nm. A filter of novel structure is fabricated by using the fused biconical taper technologies in experiment. The experiment results are in good agreement with the analytical ones.