Based　on　the　Miedema　model　and　Extended　Miedema　theory,　the　formation　energies　of　different　solute　components　in　Er-X　binary　system　and　Al-Er-X　ternary　systems　were　calculated.　The　results　show　that　the　variation　of　erbium　content　has　little　influence　on　the　chance　rate　of　the　formation　Al-Er　compound　in　the　Al-Mg-Mn-Zr-Er　system.　The　calculated　formation　energies　in　the　Er-X　binary　system　and　Al-Er-X　ternary　systems　indicate　that　Al3Er　phase　can　take　priority　of　depositing,　Al-Er-Zr　ternary　compounds　may　also　found　which　agrees　with　the　experimental　results　in　references.　The　consistency　of　calculation　and　experiment　proves　that　the　intermetallic　compounds　in　the　Al-Mg-Mn-Zr-Er　system　can　be　predicted　directly　by　calculating　the　formation　energies　of　the　reactions　in　Er-X　binary　system　and　Al-Er-X　ternary　systems　with　the　Miedema　model　and　Extended　Miedema　theory.