Gain flattening of EDFA (erbium-doped fiber amplifier) is a critical issue for WDM (wavelength-division-multiplex) system, and cost efficiency and insertion loss make in-fiber gratings very attractive candidates for this application. A chirp fiber grating to flatten the gain spectrum of L-EDFAs by using layer-peeling method is designed. This algorithm, based on the law of causality, considers fiber Bragg gratings as a discrete model, which divides the entire grating into a series of discrete, complex reflectors with a distance Δ between the adjacent reflectors, the coupling coefficient at the back of a reflector can be derived from one at the front of this reflector recursively, so coupling coefficient function can be extracted rapidly and accurately. The target reflection spectrum of chirp gratings is derived from the ideal transmission spectrum of gain-flattening filters. A constant, α, is utilized to control the length of chirp gratings, which is relative with group time delay of reflection spectrums. When α is equal to 0.0024 cm2, the grating has a short length of 3.5 cm. After extracting coupling coefficients of the chirp grating using layer-peeling algorithm, transmission spectrum of the synthesized grating by solving Riccati equation is simulated, numerical analysis indicates that peak-to-peak error function is below 0.1 dB between ideal and realistic transmission spectrums. The variation of group time delay in transmission spectrum is less than 0.6 ps over entire operation bandwidth; therefore, the grating has no impact of additional chromatic dispersion on system performance.