We demonstrated an efficient way to enhance and flatten the emission cross sections of Er3+ ions at the L-band in the silicate fiber amplifier by increasing Mg2+ (up to 22.5 mol%) with high field strength. High values of Er3+ concentration, lifetime, and L-band emission cross section were achieved in our silicate fibers. Particularly, the flatness at the L-band was achieved to be 0.8 dB, and a high gain coefficient at 1625 nm (4.7 dB/m) was demonstrated by pumping meter-scale Er-silicate fibers. The as-prepared Er-silicate fibers are attractive for the L-band fiber amplifier.