Wire-arc　direct　energy　deposition　(WA-DED)　has　emerged　as　a　suitable　solution　for　the　rapid　manufacturing　of　magnesium　(Mg)　alloys,　and　the　in-situ　improvement　of　the　microstructure　and　properties　is　essential　to　promote　its　further　application.　In　this　work,　a　novel　in-situ　high-dynamic　micro　hammering　enhanced　WA-DED　method　for　Mg　alloys　based　on　linear　actuator　was　proposed,　and　the　relevant　system　was　developed.　The　special　terminal　hammering　head　could　be　sufficiently　close　to　the　molten　pool　to　enable　high-frequency　uniform　hammering　of　the　just　solidified　Mg　alloy.　Two　hammered　layers　of　AZ31B　Mg　alloy　with　different　levels　of　deformation　(20%　and　35%)　were　fabricated　to　compare　with　the　as-deposited　layer.　The　results　showed　that　the　temperature　of　the　hammered　area　reached　450　°C　and　that　the　hammering　flattened　the　surface　of　the　deposited　layer　while　the　induced　strain　was　sufficiently　transmitted　to　the　bottom　of　the　layer.　The　formation　of　numerous　{10−12}　extension　twins　were　involved　in　strain　coordination　in　all　regions　of　the　moderately　deformed　layer　(#H-20).　Continuous　dynamic　recrystallisation　(CDRX),　twin-CDRX　and　twinning　together　contribute　to　grain　refinement,　reducing　the　average　grain　size　from　161.5　µm　to　32.5　µm.　For　severely　deformed　layers　(#H-35),　a　greater　number　of　proliferating　dislocations　led　to　a　high　degree　of　CDRX,　resulting　in　the　reduction　of　stress　concentrations　thereby　inhibiting　twinning,　and　thus　CDRX　was　the　dominant　mechanism　driving　the　microstructure　evolution,　especially　in　the　top　region.　The　microhardness　of　in-situ　hammered　samples　H-20　and　H-35　increased　by　17.7%　and　31.7%,　respectively,　and　the　grain　boundary　strengthening　and　dislocation　strengthening　both　contributed　to　the　improvement　of　mechanical　properties.　These　promising　findings　will　facilitate　the　application　of　in-situ　hammering　technology　in　additive　manufacturing　using　WA-DED.　©　2025