An interleaver with three cascaded 3×3 single-mode fiber couplers and one 2×2 single-mode fiber coupler is presented. Under certain values of physical length differences of the interferometer arms, 3-dB passband in odd channels and in even channels of the interleaver can realize an asymmetric passband wavelength response. Through a mathematical algorithm, the optimum splitting ratios, which are the relationship among the splitting ratios of the four couplers, were determined to achieve a flattop response within the passband for the interleaver. Based on the optimum splitting ratios, the output spectrums of the interleaver were analyzed and discussed. The results indicate that the amplitude of ripples in the “flat” region of the flattop passband wavelength response in odd channels and even channels are almost zero, the interleaver can achieve a perfect smoothing flattop response and good uniform channel passband, and the tolerance characteristics of the splitting ratios is favorable. In fabrication recipe, splitting ratios of the four couplers in the interleaver can be accurately controlled, so the degree of manufacturing difficulty is reduced.