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　signal-to-noise　ratio　of　the　spectral　is　greater　than　30dB,　the　polarization　extinction　ratio　is　superior　15dB　and　the　M-x(2)　and　M-y(2)　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.