We demonstrated a narrow linewidth linearly polarized all-fiber laser operating at maximum output power of 3.08 kW with a 3 dB linewidth of 0.2 nm based on a simple master oscillator power amplifier (MOPA) structure. The polarization extinction ratio (PER) is about 94% and the M² is about 1.4 along the whole power scaling process. This is the first demonstration of a 3 kW linearly polarized all-fiber laser output. Compared with the narrow linewidth fiber amplifier based on phase modulation, this system can achieve a nearly same linewidth. Meanwhile, it has the characteristics of high stimulated Brillouin scattering (SBS) threshold, simple structure, and low cost, etc.