In　this　work,　the　modification　mechanism　of　Sr　on　Si　phase　was　studied,　and　the　effect　of　Sr　element　on　the　microstructure　and　mechanical　properties　of　Al-12Si-4Cu-2Ni-Mg　alloy　was　analyzed.　The　microstructure　and　twins　of　the　alloy　were　characterized　and　analyzed　using　scanning　electron　microscopy,　electron　spectroscopy,　and　transmission　electron　microscopy.　The　results　show　that　the　twin　plane　re-entrant　edge　(TPRE)　mechanism　and　impurity　induced　twinning　(IIT)　mechanism　cannot　fully　explain　the　modification　mechanism　of　Si　phase　in　Al-12Si-4Cu-2Ni-Mg　alloy.　Various　nano-precipitates　appeared　on　the　Si　phase　of　the　as-cast　alloy,　including　a　large　amount　of　Al　rich　nano-precipitates,　Sr　rich　nano　precipitates　near　twins,　and　a　small　amount　of　AlCu　nano-　precipitates.　These　nano-precipitates　and　Sr　atoms　together　lead　to　changes　in　the　size　and　morphology　of　the　Si　phase.　With　an　increase　in　the　Si　phase　modification　degree,　the　ultimate　tensile　strength　(UTS)　and　elongation　(El)　of　the　alloy　at　room　temperature　improved.　Excessive　Sr　leads　to　the　formation　of　coarse　Al2Si2Sr,　which　is　not　conducive　to　improving　the　modification　degree　and　room　temperature　mechanical　properties　of　the　alloy.　Sr　had　no　effect　on　the　morphology,　type,　and　quantity　of　Ni-rich　intermetallic　compounds　in　the　T6-treated　Al12Si-4Cu-2Ni-Mg　alloys.　The　UTS　and　El　of　T6　alloys　at　high　temperature　are　closely　related　to　the　degree　of　modification　of　Si　phase　in　the　alloy.　The　three-dimensional　overlapping　structure　of　the　second　phase　in　the　T6-　state　Al-12Si-4Cu-2Ni-Mg-0.01　Sr　alloy　is　stable,　and　its　UTS　and　El　reach　a　peak　at　high　temperature.