In　this　paper,　we　demonstrate　the　generation　of　a　radially　polarized　beam　(RPB)　with　a　maximum　average　output　power　of　20　W　by　our　self-developed　nanosecond　pulsed　ytterbium　(Yb)-doped　fiber　MOPA　system.　The　method　of　spatial　phase　conversion　was　adopted　for　the　system,　and　a　liquid　crystal　polymer　(LCP)　vortex　half-waveplate　(LCP-WPV)　was　utilized　to　realize　efficient　conversion　of　the　linearly　polarized　LP01　mode　to　the　radially　polarized　TM01　mode.　A　narrowlinewidth,　linearly　polarized　fundamental　mode　with　an　average　power　of　20.5　W　was　produced　in　the　multistage　all-fiber　MOPA　system　consisting　of　a　continuous-wave　(CW)　single　frequency　fiber　laser　seed,　an　electro-optical　intensity　modulator,　two-stage　core-pumped　amplifiers　and　three-stage　cladding-pumped　amplifiers.　Afterwards,　an　RPB　output　with　a　20　W　maximum　average　output　power,　a　7.5　ns　pulse　width　and　a　10　kHz　repetition　rate　was　finally　obtained　after　the　LCP-WPV　was　adopted　as　the　spatial　mode　converter.　Then,　the　mode　purity　of　the　RPB　was　measured　to　be　approximately　88.5　%　with　the　PBS　(polarization　beam　splitter)　method.　This　work　provides　a　reference　for　the　generation　of　cylindrical　vector　beam　sources　with　the　advantages　of　high　efficiency　and　high　mode　purity.