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　μm,　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　©　2016　Chinese　Physical　Society.