An　all-fiber　laser　is　developed　with　high　peak　power,　narrow　linewidth,linearly　polarization,　and　nanosecond　pulse.　The　single-frequency　continuous　fiber　laser　is　externally　modulated　by　an　electro-optical　intensity　modulator(EOIM)　and　a　stable　pulsed　seed　laser　is　obtained　with　the　pulse　width　of　10　ns　and　the　repetitions　rate　of　10　kHz.　Multistage　MOPA　structure　scheme　is　adopted.After　two　stages　core-pumped　preamplifiers　and　three　stages　cladding-pumped　amplifiers,　the　average　output　power　of　the　laser　reach　to　21.3W　with　the　peak　power　of　267　kW.　At　the　highest　output　power,　the　signalto-noise　ratio　of　the　spectral　is　greater　than　30dB,　the　polarization　extinction　ratio　is　superior　15dB　and　the　Mx2　and　My2　factors　are　1.28,　1.23,　respectively.　Stimulated　Brillouin　scattering　(SBS)　was　not　found　in　the　experiment.　By　increasing　the　pump　power,　the　average　power　and　peak　power　can　be　further　improved.　©　2020　SPIE.