Based on the Giles model with ASE, modeling for all-optical gain-clamped L-band EDFA using a single fiber Bragg grating (FBG) is built. The influence of the configuration, the pump wavelength, the Bragg wavelength, the reflectivity, the length of erbium doped fiber (EDF) and the pump power to the gain-clamped characteristics is studied. Finally, a set of parameters for a gain-clamped L-band EDFA with flat-gain band is given.