Based on the theoretical model of few mode erbium doped fiber amplifier (FM-EDFA), the modal gain characteristics of uniform erbium-doped fiber are analyzed and a layered Er3+ doped fiber structure is proposed. By optimizing the Er3+ doped fiber with genetic algorithm, the equalization of four mode groups is realized. The simulation results show that,with 980 nm single-mode pump mode, the average gain of four mode groups can reach 22 dB and different modal gain is less than 0.5 dB at the wavelength of 1550 nm. The flatness of modal spectra is about 2 dB. This not only simplifies both the structure of pump mode and complexity of erbium-doped fiber manufacturing process, but also provides theoretical basis for the further research of realizing the equalization of four mode groups in experiment.