Magnesium　alloys,　renowned　for　their　outstanding　properties,　present　promising　applications　within　the　realm　of　advanced　manufacturing.　Addressing　the　challenges　of　coarse　grains　and　inferior　properties　in　the　fusion　welding　of　magnesium　alloys,　a　novel　strengthening　approach　for　fusion-welded　joints　is　proposed　in　this　study.　This　method　employs　fine,　uniform,　and　dense　grains　as　the　matrix,　with　particle-reinforced　structures　serving　as　the　strengthening　backbone.　By　utilizing　powder　feeding　in　conjunction　with　fusion　welding　and　friction　stir　processing,　ideal　microstructures　were　successfully　fabricated,　and　the　mechanisms　influencing　grain　morphology　and　joint　properties　were　explored.　Following　the　strengthening　treatment,　the　grain　size　of　the　magnesium　alloy　molten　weld　was　reduced　from　193.5　mu　m　to　15.9　mu　m,　resulting　in　a　grain　refinement　efficacy　of　91.8　%.　Concurrently,　the　tensile　strength　was　augmented　by　70.2　MPa,　and　the　elongation　experienced　a　136　%　enhancement.　The　grain　morphology,　eutectic　phase　structure,　texture　strength,　precipitated　phase　type　and　dislocation　distribution　at　each　processing　stage　of　the　welded　joint　were　observed.　Research　shows　that　nanoenhanced　particles　adhere　to　the　eutectic　structure　of　grain　boundaries,　effectively　inhibiting　the　diffusion　of　solute　elements　and　significantly　reducing　the　dendrite　growth　rate.　It　is　the　main　mechanism　by　which　reinforced　particles　lead　to　grain　refinement.　Discontinuous　recrystallization　during　FSP　treatment　leads　to　grain　refinement,　work　hardening　and　enhanced　pinning　of　particles　at　grain　boundaries,　which　are　the　main　reasons　for　the　strength　improvement.