An all polarization-maintaining fiber femtosecond laser amplification system, using Yb-doped large-mode-area double-cladding fiber (LMA-DCF) as gain medium and adopting the chirped pulse amplification technology, was constructed. Benefiting from the all polarization-maintaining fiber construction, the amplification system showed high -integration and long-term stability. Furthermore, the quality and duration of the pulses after compression were optimized by controlling the length of the stretcher fiber, which made it possible to compensate the third-order- dispersion accumulated in the system by introducing nonlinear chirp in the amplifier. Meanwhile, we analyzed the impact on amplified pulses caused by periodic spectral modulation. Through optimizing artisan craftsmanship of polarization-maintaining fiber (2+1)×1 combiner, the issue of periodic spectral modulation was solved. Finally, the high-quality ultrashort laser pulses with an average power of 9 W and a pulse duration of 260 fs at the repetition rate of 111 MHz after compression, corresponding to single pulse energy up to 81 nJ and pulse polarization ratio of 13 dB, were generated.