The influence of the additions of Mg-Zn-Y quasicrystals-containing master alloy on the microstructures and mechanical properties of AZ91 alloy under conventional casting condition was studied using XRD, SEM equipped with energy dispersive spectrometer (EDS). The results show that the microstructure of Mg-Zn-Y quasicrystals reinforced AZ91 alloy consists of α-Mg supersaturating solid solution, β-Mg17Al12 phase and quasicrystals phase. Quasicrystals particles with excellent elevated temperature stability are dispersively distributed in the α-Mg matrix or at grain boundaries. After the addition of quasicrystals-containing master alloy, the matrix microstructure of AZ91 alloy is obviously grain-refined. The morphology of β-Mg17Al12 phase changes from continuous nets to discrete nets. At room and elevated temperatures mechanical properties of AZ91 alloy are also improved dramatically. The utility of Mg-Zn-Y quasicrystals as a reinforced phase provides new theoretical basis and technical support for the composite strengthening of magnesium alloys.