To improve the utilization efficiency of bandwidth in an interleaver system,an all-fiber different bandwidth optical interleaver based on a cascaded Mach-Zehnder interferometer(MZI) is proposed an d discussed in this paper.The all-fiber optical interleaver consists of one plan ar 3×3and one 2×2fiber couplers. Based on its structure,the output spectrum expression is established and descri bed by using the principle of fiber transmission and the matrix transfer function.According to the Fourier se ries digital approximation method,influences on its transmission characteristics by such factors as splitt ing ratio of the coupler of the directional fiber couplers and the length differences of the interferometer arms are numerically analyzed and optimized.The results of numerical simulation indicate that when the coupling c oefficients of the couplers and the length differences of interferometer arm are some certain values,50GHz inpu t single of device is separated into channel spacing of 100GHz odd-even two output signals.Their spectra bandw idth is 31.5GHz and 65.6GHz at 3dB,it satisfies the requirement of different transmission rates on bandwidth in 10Gbit/s plus 40Gbit/s mixed system and implements different bandwidth transmission,it has hi gher bandwidth utilization that compared with the equivalent bandwidth interleaver.The device has a certai n ability to resist the deviation.Compared with the ordinary cascaded MZI interleaver,the most advantage of the new interleaver is that reduced numbers of coupler.With the configuration,the coupling ration of couplers can be controlled accurately,which reduces the difficulty of making it.The device in 10Gbit/s plu s 40Gbit/s optical fiber communication system has a potential application value.A novel structure of int erleaver is fabricated with continuous melting method in experiment.The experimental results agree with the theoretical analysis well.