A novel method for designing the interleaver applied in DWDM optical communication is presented. This passive component combines the nonlinear phase feature of the reflective beam within F P cavity and the linear periodic phase characteristic of the beam at certain propagation difference with the frequency. The performance could satisfy the DWDM′s requirement when those two phase matched beams interfered. The theoretic model is simulated using the characteristic matrix of thin film. The result is calculated and analyzed, and the feasibility of this kind of design is demonstrated.