Characteristics of gain spectral variation of EDFs and its dependence on aluminum doping level and fiber mode design are quantitatively studied. Based on experimental data and manufactured fibers with different aluminum levels, the correlation between aluminum concentration and both absorption spectrum and gain flatness is revealed. Gain spectral variation for over a million meters of EDFs manufactured in last several years is presented. The result shows that peakto-peak spectral shape variation for all these fibers are within 0.8% in a 36nm C-band window.