In broadband (～35 nm) EDFAs, gain flatteningfilter can be inserted between the two stages of EDFs. In this paper, an iterationalgorithm for designing the filter′s transmission function is proposed. The principle of thisalgorithm is taking the advantages of homogeneous broadening and gain compensation betweentwo stages. In each iteration step, the average gain level is taken as reference, andwithin the wavelength range where the gain is higher than the reference, an inverse gainspectrum is used to modify the transmission function. Then the gain within otherwavelength range will be compensated automatically. Simulation results show that by usingthis iteration algorithm, the flattened gain can reach the possible highest value of theoriginal amplifier, and noise penalty is the minimum.