Abstract: An optimization calculation of interleaver made of cascade Mach-Zehnder fiber interferometer with square wave is put forward. Based on transmit-matrix, the output spectrum from the cascade M-Z interferometer can be expressed as explicit function, the square wave output function of perfect interleaver can be expanded in a unique recursion way. Sometimes there is no accurate outcome when the terms of the two functions are compared, but approximate structureparameter of cascade Mach-Zehnder fiber interferometer can be obtained by optimizing calculation method with Matlab, which can make the output-spectrum an approximate square wave. The design example indicates that this method is convenient, flexible and effective.