The　microstructures　of　Al-20%Si　(mass　fraction)　alloy　prepared　by　melt　thermal　treatment　(including　the　melt　mixing　and　melt　superheating)　were　studied　combining　with　chemical　metamorphism　to　further　refine　the　primary　silicon.　The　results　show　that,　when　the　mixed　melt　is　superheated　to　900°C,　the　grain　size　of　primary　silicon　is　decreased　to　about　34　μm.　And　adding　the　modificator　or　refiner　into　the　melt　before　melt　mixing　can　further　refine　the　primary　silicon　of　Al-20%Si.　Especially,　when　the　Al-10%Si　melt　mixes　with　the　Al-30%Si　melt　added　0.2%Al-5Ti-C-3Ce　and　0.4%Cu-10P,　respectively,　then　superheated　the　mixed　melt,　the　primary　silicon　of　Al-20%Si　is　well　uniformly　distributed　in　tiny　blocklike　with　the　grain　size　less　than　10　μm,　which　makes　the　matrix　of　the　alloy　exhibit　typical　composite　characteristic.　The　analysis　shows　that　the　melt　thermal　treatment　and　adding　modificator　or　refiner　at　the　same　time　have　multiple　modification　effects　on　the　primary　silicon.　The　melt　thermal　treatment　increases　the　supercooling　degree　during　the　solidification　process　of　the　alloys　to　refine　or　modify　the　primary　silicon　because　of　the　re-melt　of　primary　α(Al)　and　the　recombination　of　chemical　bond　in　Al-Si　alloy　as　mixing　the　different　melt　together.　And　then　superheating　the　mixing　melt　can　make　the　pre-crystallized　primary　silicon　split　and　proliferate,　thus　further　refine　or　modify　the　primary　silicon.　And　adding　refiner　or　modificator　can　improve　the　refinement　effect　of　the　melt　thermal　treatment　on　the　primary　silicon　of　the　Al-Si　alloys.