The transmission function of the Interleaver consisting of two cascaded unbalanced Mach-Zehnder interferometer is analyzed. Based on the Fourier series digital approximation method and trapezium wave which is utilized to simulate the output waveshape, the optimal parameter of the Interleaver has been studied.This kind of Interleaver has flat pass band and broad cut band, so it's stability and isolation are improved greatly. The Interleaver is also studied in experiment. The theoretical results are consistent with the experimental results.