A two-stage optical amplification system is constructed by using double-clad Yb-doped fiber as the gain medium and multi-mode continuous pumped laser diode as the pump source.The circuit-direct modulated semiconductor laser is used as seed source to develop Master-Oscillator Power-Amplified（MOPA）fiberized pulsed fiber lasers.The output power characteristics of the fiber laser and gain characteristics of the secondary amplifier under different pulse width and repetition frequency are studied experimentally.A pulsed laser output with central wavelength of 1.06μm

peak power of 10.4 kW and maximum average power of 21.3 Wis obtained by adjusting the seed source under the condition that the primary pumped power is 2 W and secondary pumped power is 31 W.The maximum gain can reach 16.7 dB at 900 kHz repetition frequency.The experimental results show that the pulse width and repetition frequency are positively correlated with the laser output power.At high repetition frequency

the gain characteristics of optical amplifier are positively correlated with the pulse width.