Abstract: In order to investigate gain transient performance of burst-mode high-power Er/Yb co-doped fiber amplifier (EYDFA) in optical packet and circuit integrated network, comparison mode of gain transient performance of EYDFA with different doping area was proposed by adopting iterative algorithm of nonlinear rate-equations based on function limit. Gain transient performance of optical packet switching and optical circuit switching output signals with/without optical feedback , are analyzed and compared in detail. The results show that the maximum improvements of gain excursion for EYDFAs without optical feedback are about 12% when the core diameter of double-clad fiber changes from 3.6m to 4.6m. The gain fluctuation of EYDFAs with optical feedback caused by relaxation oscillation decreases with the increase of core diameter. The analysis suggests that relaxation oscillation of EYDFA feedback loop will be suppressed by enlarging active core diameter of doped fiber.