This paper proposed a novel method to design Interleaver,which was implemented by single stage M-Z interferometer with ring waveguides.Compare the power transmittances among the third grade interleaver,the fifth grade interleaver and the seventh grade interleaver,it is found that with the grade of interleaver adding flat-top effect is better,the steeper spectral responses of the stopband and passband are achieved,and power transmittances get close to square wave.The merits of the design is that if only the transfer function of desired cross port for elliptic half-band filter is determined,circuit parameters can be gained and flat-top spectral response and power transmittance closing to square wave are achieved by adding the grade of interleaver.