The theory investigation of a flat-top optical interleaver with two ring cavities in a M-Z interferometer is present. We find the optimum coupling angle of the two two ring cavities which meet the needs for DWDM. The result indicates that the interleaver with two ring cavities can simultaneously produce a 0.5 dB passband and a 25 dB stopband wider than with only one ring cavity. Influence on its transmission characteristics by the coupling angle is also analyzed.