A novel constructure and theory of the optical waveguide filter are presented in this paper. The optimum design of the filter can lead to obtaining the size of the construoture directly. An optical waveguide filter with maximum flat response at 1.55 μm of central wavelength and 22 nm of bandwidth is obtained. The calculation in the design is simple and can be used to the filter construction. Since the design takes the geometry discontinuity of each guide into account, the accuracy of the results is better than that by EDC method.