The　valence　electron　structure　of　Al-Er　alloys　were　calculated　according　to　the　empirical　electron　theory　in　solid　(EET).　And　equilibrium　solubility　of　Er　in　pure　Al　at　different　temperatures,　distribution　problem　of　solute　atom　Er　during　the　solidification　was　analyzed　based　on　the　redistribution　mechanism　in　the　interface.　The　results　demonstrated　that　the　covalence　electron　number　of　the　strongest　bond　in　Al3Er　cell　was　0.35798,　which　indicated　that　there　was　a　strong　interaction　between　Al　atoms　and　Er　atoms.　So　Al　and　Er　would　tended　to　form　the　Al3Er　phase.　The　relationship　of　the　strongest　bond　energy　in　crystal　cell　of　solid　was　EEr　>　EAl-Er　>　EAl.　As　a　result,　the　strength　and　high　temperature　resistance　of　pure　Al　was　improved.　The　equilibrium　solubility　of　Er　in　pure　Al　was　as　low　as　0.017%　at　the　eutectic　temperature,　and　the　equilibrium　solid-liquid　partition　coefficient　K=0.0028　which　was　far　less　than　1.　According　to　Sheil　model,　a　great　composition　supercooling　would　occur　even　though　the　content　of　Er　was　very　low,　the　Al3Er　phase　formed　in　the　solid-liquid　interface　when　the　content　of　Er　was　up　to　the　eutectic　composition.　Thus　Al3Er　phase　acted　as　heterogeneous　nuclear　which　was　firstly　crystallized　in　alloy　to　make　grains　refine　remarkably.