An all-fiberized master oscillator power amplifier(MOPA) structured pulsed Yb fiber laser is reported. The seed source was an acousto-optic (AO) Q-switched Yb fiber laser working at 1064 nm and emitting a random polarized pulsed emission with an average power of 2 W at repetition rates between 20 kHz and 50 kHz. Using a polarization maintaining(PM) large mode field double-cladding fiber as the gain medium and six LDs each with 10 W power output at 915 nm as the pump, the Yb fiber amplifier delivered a linearly polarized pulsed emission output at 1064 nm with an average power of 23.5 W, pulse duration about 30 ns, polarization extinction ratio greater than 10 dB, and a beam quality factor M2 of 1.36. The effect of mode field mismatch between the output fiber of seed laser and the large mode field PM fiber, which influenced the beam quality and the spectral distribution of the laser output, was discussed.