Abstract: In this study, a theoretical model of reflective filter with fiber grating is analyzed. The model composed of sampling fiber grating and fiber directional coupler has the function of multi-wavelengths chosen and optical isolation. The mathematical formulation of the relationship between the lights input and output is educed from the method of transformation matrix. The parameters and rules that decide spectra characteristic of the filter output and the function of optical isolation are discussed in detail and emulated. The structure parameters of this model that realize pectination filtering and optical isolation simultaneously are given. This conclusion can provide reference value for later manufacturing of this devices and solution to key technology.