Abstract: The dynamics characteristics of a mode-locking laser using a gain-flattened erbium doped fiber amplifier are reported.Based on polarization additive pulse mode locking technique,a self-starting mode-locking of the fiber laser was demonstrated with a gain-flattened erbium doped fiber amplifier,fiber polarization controllers and a polarization dependent isolator.The stable optical pulses train at 1560nm center wavelength and 6.495MHz repetition rate was generated,with 1.5ps pulse width and 0.7nJ pulse energy.Mode locked pulses with dual peak-wavelengths at 1557nm and 1570nm were also observed.The results of the experiment indicate that higher energy of the pulses can be generated by the fiber laser with a gain-switched erbium doped fiber amplifier than that with a general erbium doped fiber.The mode-locked pulse spectrum of the fiber laser with a gain-flattened erbium doped fiber amplifier could show dual peak structure.It's valuable to the design and practicability of the ultra-short fiber lasers.