A linearly polarized single-transverse-mode Yb-doped double clad polarization-maintaining fiber amplifier at 1064nm is reported, which delivers 53.1W average power at 40kHz repetition rate, with pulse energy >1mJ, duration 30ns, polarization extinction ratio 13dB, and M(2)1.2. Stimulated Brillouin scattering (SBS) phenomenon during amplification is discussed, and a new SBS suppression approach using linewidth broadening induced by self-phase modulation (SPM) is demonstrated.