We　demonstrate　high　average　power　all-fiber　single-polarization　thulium(Tm)-doped　nanosecond　laser　based　on　all-fiber　master-oscillator　power-amplifier　(MOPA).　The　seed　source　employed　is　a　directly　modulated　discrete-mode　(DM)　diode　at　2　mu　m　with　pulse　width　of　15.8　ns　and　tunable　repetition　rate　range　of　400　kHz　to　4　MHz.　The　pulse　duration　reduced　to　6　ns　as　the　gain　reshaping　during　amplification.　The　MOPA　yields　160　W　maximum　average　powers　at　4　MHz　repetition　with　6　ns　pulse　width,　with　respect　to　pulse　energy　of　40　mu　J　and　peak-power　of　6.7　kW.　The　slope　efficiency　for　the　last　stage　of　PM　LMA　Tm-doped　fiber　amplifier　was　48.8%　with　respect　to　launched　pump　power.　The　polarization　extinction　ratio　(PER)　of　system　measured　at　maximum　output　power　is　beyond　15　dB.　The　beam　quality　factor　M-2　was　measured　to　be　1.5　at　average　output　power　of　160　W.　To　the　best　of　our　knowledge,　this　is　the　highest　average　power　2　mu　m　nanosecond　pulses　laser　from　laser　diode　seeded　all-fiber　MOPA　systems　to　date.