A cascaded Mach-Zehnder interferometer filter is used to construct a flat-top filter, but couplers' coefficients of the cascaded interferometer-based filter cannot be determined directely by the traditional transfer matrix way. An analogy between the cascaded Mach-Zehnder interferometer-based filter and the finite impulse response filter used in the signal processing is given, the transfer function of the filter can be explicitly expressed by the couplers' coefficients. Thus, some methods commonly used in digital filter design can be used to design the cascaded Mach-Zehnder interferometer-based filter. For a specified frequency response, the couplers′ coefficients of the cascaded interferometer-based filter are determined by the genetic algorithm. An example is given to verify the availability and efficiency of this method.