The　growth　technology　of　AlGaN　film　with　high　Al　composition　and　large　thickness　is　investigated.　The　phase　separation　of　AlGaN　in　AlGaN/GaN　structure　may　be　due　to　the　too　low　growth　of　V/III　and　the　tensile　stress.　Meanwhile,　with　ultra-high　V/III,　the　Al　incorporation　rate　can　be　saturated.　By　adopting　AlN　interlayer,　no-crack　and　thick　AlGaN　materials　with　45%　Al　composition　are　grown.　The　Al　composition　of　this　sample　is　the　same　as　that　in　gas　phase　and　the　width　of　half　height　of　the　rocking　curve　of　(0002)　plane　is　376″.　It　is　found　that　Al　composition　and　crystal　quality　of　the　AlGaN　film　are　influenced　by　the　quality　of　AlN　interlayer.　It　is　also　seen　that　as　the　Al　composition　increases,　the　morphology　of　AlGaN　materials　changes　from　micropit　mode　to　microcrack　mode,　which　can　be　retarded　by　the　AlN　interlayer.