In　this　work,　we　demonstrate　a　gain-managed　nonlinear　amplifier　with　a　large　mode　area　fiber　numerically　and　experimentally.　The　seed　source　was　a　home-made　mode-locked　fiber　oscillator　using　a　nonlinear　amplifying　loop　mirror　(NALM)　that　delivered　ultrashort　pulses　with　a　pulse　width　of　8.77　ps　at　a　repetition　rate　of　1　MHz.　The　corresponding　optical　spectrum　centered　at　1029　nm　with　a　3dB　bandwidth　of　13　nm.　The　pulse　energy　was　increased　to　5　nJ　by　a　core-pumped　fiber　pre-amplifier.　After　then,　a　narrow-band　spectral　filter　was　inserted　to　reduce　the　pulse　width　and　injected　into　a　3　m　long　double-clad　Yb-doped　fiber　with　a　core　diameter　of　20　μm,　which　was　forward　pumped　with　a　976　nm　laser　diode.　Finally,　we　obtained　1.33　μJ　pulses　energy　delivering　after　amplification,　and　50　fs　pulses　output　was　obtained　by　post　compression　through　600　line/mm　reflective　grating　pairs.　The　numerical　work　has　been　carried　out　as　well,　which　has　good　agreement　with　the　experimental　result.　This　highly　compact　femtosecond　fiber　laser　system　with　an　ultrashort　pulse　width　has　potential　applications　in　scientific　research　and　precision　manufacturing.　©　2023　SPIE.