The　work　aims　to　improve　the　elongation　of　Ti6Al4V　alloy　formed　by　selective　laser　melting　(SLM),　so　that　the　tensile　properties　of　the　formed　structure　can　approach　or　meet　the　forging　standard.　The　in-situ　decomposition　method　was　used　to　decompose　the　acicular　a’　martensite　in　the　formed　layer　into　a+Β　phase　by　adjusting　the　process　parameters　such　as　laser　power,　energy　density,　thickness　of　the　layer　and　the　area　ratio　of　the　support　in　the　SLM　forming　process　under　the　thermal　cycle　of　heat　source　reciprocating　motion　for　reciprocating　heating　and　cooling.　By　means　of　microstructure　observations　(SEM),　phase　analysis　(XRD)　and　tensile　test,　the　conditions　for　in-situ　decomposition　of　Ti6Al4V　alloy　were　determined.　Increasing　the　thickness　of　SLM　layer　(60　µm)　and　laser　power　(375　W)　was　beneficial　to　reducing　the　cooling　rate　and　temperature　gradient　in　SLM　manufacturing　process,　resulting　in　α′→α+β　transformation　of　martensite　structure.　The　results　of　SEM　and　XRD　showed　that　the　morphology　of　Ti6Al4V　alloy　after　in-situ　decomposition　was　composed　of　acicular　a　phase　and　granular　Β　phase,　which　was　different　from　that　of　conventional　SLM　at　high　temperature　gradient　and　extremely　fast　cooling　rate.　The　tensile　test　results　showed　that　the　elongation　of　Ti6Al4V　specimens　increased　while　high　yield　strength　was　maintained　after　in-situ　decomposition.　The　yield　strength　reached　more　than　1100　MPa,　and　the　elongation　reached　8%.　The　fracture　mechanism　was　ductile　fracture.　SLM-formed　Ti6Al4V　alloy　has　better　toughness　after　in-situ　decomposition,　which　can　improve　the　tensile　properties　of　Ti6Al4V　alloy.　©　2019,　Chongqing　Wujiu　Periodicals　Press.　All　rights　reserved.