We　demonstrate　an　all-fiberized　narrow-linewidth　nanosecond　amplifier　with　high　peak　power,　tunable　pulse　width,　and　repetition　rate.　A　fiber-coupled　narrow-linewidth　laser　diode　operating　at　1064.1　nm　is　employed　as　the　seed　source,　which　is　gain-switched　to　generate　nanosecond　pulses　with　tunable　pulse　widths　of　1-200　ns　and　tunable　repetition　rates　of　10　Hz-100　kHz.　By　utilizing　a　very-large-mode-area　Yb-doped　fiber　with　a　core　diameter　of　50　mu　m　in　the　power　amplifier,　thresholds　of　the　stimulated　Brillouin　scattering　at　different　pulse　widths　and　repetition　rates　are　increased.　The　maximum　average　power　reaches　30.8　W　at　the　pulse　width　of　4　ns　and　a　repetition　rate　of　100　kHz,　corresponding　to　an　optical-to-optical　conversion　efficiency　of　similar　to　55.2%.　Pulse　energy　and　peak　power　are　calculated　to　be　0.2　mJ　and　50　kW,　respectively,　which　are　limited　by　stimulated　Brillouin　scattering.　The　3-dB　spectral　linewidth　remains　around　0.05　nm　during　the　power　scaling　process.　The　stimulated　Brillouin　scattering　limited　output　powers　at　different　pulse　widths　and　repetition　rates　are　investigated.　Peak　power　of　47.5　kW　(0.19　mJ)　is　obtained　for　the　4　ns　pulses　at　a　repetition　rate　of　50　kHz,　which　is　nearly　the　same　as　that　of　4　ns　pulses　at　100　kHz.　When　the　pulse　width　of　the　seed　source　is　increased　to　8　ns,　peak　powers/pulse　energies　are　decreased　to　19.6　kW/0.11　mJ　and　13.3　kW/0.08　mJ　at　repetition　rates　of　50　kHz　and　100　kHz,　respectively.