High-power narrow-linewidth rare-earth-doped fiber lasers, which are well known for their high beam quality and high efficiency properties, have rapidly developed in the last decade, due to the needs of a vast range of applications such as nonlinear frequency conversion, and incoherent spectral beam combination to further scale up the total output power of fiber lasers. At the same time, many efforts have also been made to extend the operating wavelength of narrow-linewidth fiber laser toward the longer mid-infrared wavelength region, which was motivated by a large number of promising applications such as atmosphere monitoring, and pump source for mid-infrared optical parametric oscillator. In most cases, thulium-doped fiber lasers operate efficiently in a wavelength range of 1.8-2.1 m, which could be considered as being one of the most important sources of narrow-linewidth laser radiation that has been developed and intensively investigated in the last several years. Here, we demonstrate a high-power narrow-linewidth continuous-wave thulium-doped all-fiber laser based on master-oscillator power-amplifier (MOPA) configuration. The MOPA yields 342 W of narrow-linewidth laser output at the central wavelength of 2000.3 nm with a 3-dB spectral bandwidth of 90 pm. The beam quality factor is measured to be M2 of 1.15 at an output power of 300 W. No indication of stimulated Brillouin scattering could be observed at the highest output power level, and the output power is only currently limited by 793 nm available pump power. This kind of high-power narrow-linewidth thulium-doped all-fiber MOPA represents a promising achievement in the generation of high-power laser source via incoherent spectral beam combination.