Asymmetrical birefrigent crystal interleaver is designed by using digital signal processing theory. Based on Jones-matrix theory, the expression of the system composed of several cascaded crystal wave plates located between a polarizer and an analyzer is calculated, and it is simplified to make the output frequency response satisfy the expression of FIR by using Z transform. Using FIR digital filter design technique, an asymmetrical interleaver is designed. The azimuth angles of crystal and analyzer are determined by using step-down iteration method based on the expression of spectral transmittance. An asymmetrical 50 GHz interleaver is designed, the ratio of one output port (narrow bandwidth) to another output port (wide bandwidth) at 3 dB passband is 1∶2. The design examples of three crystals structure using Equiripple method are given. The bandwidth at 0.5 dB passband,channel isolation and gradient in both wide port and narrow port are given. Three design results respectively got by three different FIR design methods in the same order are compared, and the advantages and drawbacks of different methods are analyzed.