The atomic structure models of α matrix and the symmetric ［0001］ tilt boundary in AZ91 magnesium alloy were set up by using the concept of coincidence-site lattice (CSL). The total structure energy of α matrix and the grain boundary (GB)，the environment sensitive embedding energies (EESE) of alloy elements and the interaction energies between the atom clusters were calculated by using recursion method. Calculation results show that the cluster formed by Al and RE in bulk AZ1 Magnesium alloy is rather stable，but the cluster of Al，Bi(or Sb) and RE atom is unstable. When Bi(or Sb) and RE atoms coexist in AZ91 magnesium alloy，Bi or Sb will combine with RE to form dispersed particles which are mainly RE2Bi (or RE2Sb ) and RE-Sb(or RE-Bi) distributed at the grain boundary，and Al11RE3 will form in the bulk in AZ91 magnesium alloy. The consumption of Al in magnesium alloy inhibits the formation of the discontinuous precipitate phase Mg17Al12. The melting points of compounds formed by RE and Al (or Bi，Sb) are high，thus the high temperature performance of AZ91 magnesium alloy would be obviously improved.