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.