A　high　power　all-fiber　nanosecond-pulsed　thulium-doped　fiber　master　oscillator　power　amplifier　(MOPA)　system　that　consists　of　low　power　passively　Q-switched　thulium-doped　fiber　laser　seed　and　two-stage　thulium-doped　double-clad　all-fiber　amplifiers　pumped　by　high　power　multimode　diodes　at　790　nm　is　studied.　The　repetition　rate　of　the　thulium-doped　fiber　laser　seeder　is　tunable　from　77　kHz　to　240　kHz　with　the　increase　of　pump　power,　the　minimum　pulse　width　is　measured　to　be　250　ns　and　the　maximum　average　output　power　is　73　mW.　Two-stage　double-clad　thulium-doped　all-fiber　amplifiers　are　used　to　boost　average　power　to　20　W,　corresponding　to　single　pulse　energy　of　100　μJ.　The　pulse　width　is　270　ns,　and　laser　center　wavelength　is　1966　nm.