We demonstrate a high-power, linearly polarized all-fiber amplifier with a narrow linewidth and near-diffraction-limit beam quality by suppressing the stimulated Brillouin scattering and self-pulsing effects simultaneously. The maximum laser power is 2.62 kW with 86.7% optical-optical efficiency. Beam quality (M²), polarization extinction ratio (PER), and linewidth are well maintained during power amplification with M²<1.3, PER ~96.3%, a 3 dB linewidth of 32 GHz, and a 20 dB second-order moment linewidth of 30 GHz. To the best of our knowledge, this is the largest demonstration of an all-fiber laser with narrow linewidth, linear polarization, and beam quality near the diffraction limit.