A simple model to analyse the influence of the random change of waveguide width caused by fabrication error on the spectral character of fluorinated polyimide Mach-Zehnder (M-Z) waveguide interleaver is presented in this paper. The working parameters of fluorinated polyimide Mach-Zehnder waveguide interleaver are the central wavelength of 1550 nm, wavelength interval of 0.8 nm, and 40 channels. The simulation calculation indicates that the main influence is the increase of the crosstalk among channels, the crosstalk of the central wavelength of 1550 nm is decreased from -40 dB to -12 dB. Based on this result, we propose a new structure to improve the crosstalk performance of the device by cascaded micro-ring resonator filters, and then the crosstalk of 40 channels was decreased to less than -30 dB.