Erbium-doped fibers(EDFs) with multi-step ormulti-annular and multi-level doping profiles are difficult to be fabricated by modified chemical vapor deposition (MCVD). A cascaded amplifier, based on two EDFs with a central doping profile and an annular doping profile, is proposed to equalize modal gain among four linearly polarized modes. In the amplifier, the centered- and off-centered modes are mainly amplified in the central- and annulardoped EDFs, respectively. An improved genetic algorithm (GA) is employed to optimize the optimal fiber length and doping radius in each EDF. Results show that an average modal gain around 20 dB with differential modal gain (DMG) between signal modes being less than 3.5 dB is achieved for a pure LP11 pump beam.