The atomic structure model of symmetric ［0001］tilt boundary of α phase in AZ91 magnesium alloy was set up by using the concept of coincidence-site lattice (CSL). The total structure energies of α matrix and grain boundary (GB), the environment sensitive embedding energies (EESE) and the interaction energies of Al, Bi and Sb were calculated by using recursion method. The alloying behaviour of Al, Bi, Sb in AZ91 magnesium alloy was discussed. Calculation results show that the total structure energies of α matrix and grain boundary (GB) are decreased due to the presence of Al, Bi or Sb, which suggests AZ91 alloy is strengthened due to solid solution strengthening. Alloying elements tend to be distributed uniformly in α matrix, and are apt to occupy the top sites of the trigonal prism formed by Mg in the GB core. The aggregation of Bi or Sb to GBs,being easier than Al, suppresses the aggregation of Al, which accelerates the formation of continuous precipitates, improves the high temperature properties. The major alloying element Al and the additional elements Bi or Sb in small amounts can form the ordered phases in AZ91 alloys: Mg１７Al１２, Mg3Bi2 and Mg３Sb２, respectively, and the quantity of the ordered phase is greater in GBs than in α(Mg) matrix. Bi or Sb additions to AZ91 retards the aggregation of Al, and Mg3Bi2 or Mg３Sb２ precipitates along boundaries greatly improve the high temperature properties.