We built a numerical model for evaluating the coupling processes of a mixed structure of a Bragg fiber grating and a long-period grating. From the numerical results, we not only confirmed the wavelength switching phenomena observed in previously reported experiments, but also discovered a new coupling mechanism, which generated the reflection of a signal with its wavelength longer than the Bragg wavelength. The dependencies of the wavelength switching behaviors on various parameters of the mixed grating structure were demonstrated. Such results should be useful for optimizing the design of such a potentially useful fiber component.