During　welding,　the　vibration　effect　of　applying　a　pulse　current　on　the　molten　pool　can　effectively　improve　weld　formation　and　refine　grains.　The　effect　of　pulse　current　on　grain　refinement　and　its　mechanism　were　studied　for　Al-Mg　alloy　welds　fabricated　by　conventional　plasma　welding　(PAW),　PAW　with　conventional　pulse　current,　and　PAW　with　composite　pulse　current.　The　grain　size　produced　by　conventional　PAW　was　78.2　mu　m,　whereas　the　average　grain　size　was　reduced　from　78.2　mu　m　to　53.3　mu　m　with　increasing　conventional　pulse　current　frequency　from　0　Hz　to　100　Hz;　in　addition,　the　degree　of　grain　refinement　increased　by　about　30%.　However,　the　minimum　grain　size　was　48.2　mu　m,　and　the　grain　refinement　effect　can　reach　nearly　40%　by　combining　low-frequency　pulse　current　with　conventional　pulse　current.　The　proportion　of　small　grains　and　high-angle　grain　boundaries　increased　significantly　after　applying　the　composite　pulse　current.　The　additional　oscillation　effect　of　the　composite　pulse　current　can　effectively　eliminate　coarse　grains　during　the　solidification　of　the　weld　pool.　The　main　mechanism　of　grain　refinement　is　dendrite　fragmentation,　which　is　discussed　through　thermodynamics　and　composition.