Three　common　issues　in　magnesium　alloy　laser　welding　are　poorly　formed　weld-seams,　coarsen　grains　and　deteriorated　quality　of　joints.　A　novel　process　to　weld　magnesium　alloys　by　laser　deep　melting　is　proposed,　and　it　synergizes　the　effect　of　the　circular　spot　of　fiber　laser　and　the　sinusoidal　modulation　of　power.　The　impact　of　power　modulation　parameters　on　surface　formation,　microstructure,　and　the　mechanical　properties　of　butt　joints　has　been　investigated　thoroughly,　and　AZ31B　is　used　as　the　welding　materials　in　investigation.　By　comparing　the　welds　from　a　circular　spot　laser　with　constant　power,　the　investigation　finds　that　the　proposed　process　has　improved　surface　collapse　in　weld　seam　significantly,　the　welds　are　formed　uniformly　when　a　circular　spot　laser　with　a　central　beam　power　modulation　is　applied.　Moreover,　a　low　fluctuation　of　the　speed　of　molten　metal　on　welded　surface　has　produced　more　stable　keyholes　and　fish-scale　patterns　on　the　welds　uniformly.　It　is　found　that　central　beam　power　modulation　also　improves　the　microstructure　of　welds.　By　setting　a　modulation　frequency　appropriately,　the　zone　with　fine　equiaxed　crystals　has　been　widened　in　the　center　of　welds,　and　the　zone　with　coarse　crystals　in　the　heat-affected　zone　has　been　reduced.　However,　when　the　modulation　frequency　is　too　high,　the　zone　with　equiaxed　crystals　is　eliminated　in　the　center　of　weld,　and　it　forms　numerous　coarse　crystals　and　some　grains　with　internal　deformation.　When　the　amplitude　and　frequency　of　power　modulation　are　as　500　W　and　200　Hz,　respectively,　the　tensile　strength　of　joint　reaches　233.41　MPa,　and　the　corresponding　elongation　rate　is　9.52%;　these　welding　properties　are　approximately　82.3%　and　76%　of　the　base　material,　respectively.　The　failure　modes　are　primary　ductile　fractures　globally　and　secondary　intergranular　fractures　in　locally.　©　2025　Chinese　Mechanical　Engineering　Society.　All　rights　reserved.