For　selective　laser　melting　(SLM)　forming　of　Ti6Al4V,　a　three-dimensional　(3D)　mesoscopic　model　of　random　distribution　of　powder　particles　was　established　based　on　the　discrete　element　method　(DEM).　The　volume　of　fluid　method　(VOF)　was　used　to　track　the　3D　dynamic　free　surface　in　SLM　forming　process.　Various　factors　were　considered　in　the　numerical　model,　such　as　the　TC4　powder　bed　with　randomly　distributed　particles,　the　thermophysical　parameters　changing　nonlinearly　with　temperature,　the　free　surface　evolution　of　the　molten　pool,　the　surface　tension　caused　by　temperature　gradients,　and　the　evaporation　effect.　The　heat　transfer,　melting,　flow　and　solidification　in　the　interaction　between　laser　and　powder　particles　were　studied　according　to　the　numerical　simulation.　Results　show　that　the　Marangoni　convection　induced　by　temperature　gradient　and　surface　tension　gradient　is　the　main　factor　affecting　the　heat　and　mass　transfer　within　the　melt　pool　and　the　3D　morphology　of　the　melt　pool.　The　line　energy　density　(LED)　is　positively　correlated　with　the　Marangoni　effect.　The　quality　of　single　track　surface　was　good　when　the　optimized　LED　ranged　from　92.9　J/m　to　183.0　J/m.　The　three-dimensional　size　and　the　morphology　of　the　molten　pool　and　the　molten　track　were　observed　and　analyzed　by　the　single　track　forming　experiment,　by　which　the　numerical　results　were　validated.　©　2019,　Zhejiang　University　Press.　All　right　reserved.