In order to improve the transmission performance of the all-fiber 3×3 interleaver, the output expression is derived and simulation is performed. Through the analysis and optimization, the optimal structure parameters is put forword. The calculating and experimental results indicate that the crosstalk characteristics, the channel isolation, stopband rejection, passband flat-topped degrees and the rolloff in transition band can be improved greatly, and the reduced sidelobe level of channel of the proposed device is more than 16 dB in comparison with that of the convention all-fiber 3×3 interleaver. A 100 GHz three-port interleaver with a flat-topped passband bandwidth is demonstrated by experiment. The experimental and theoretical results coincide with each other.