Abstract: Heat-resistant magnesium alloys Mg-9Al-2.25Sr with different Y contents were prepared in pit type tempering resistance furnace, and effects of Y contents on microstructure and mechanical properties of Mg-9Al-2.25Sr alloys were studied by optical microscopy, scanning electron microscopy, energy diffraction spectroscopy and X-ray diffraction. The results show that Y could refine the microstructure of Mg-9Al-2.25Sr alloys and combine with Al to generate granulate and dispersive Al2Y to restrain low-temperature phase β-Mg17Al12 precipitation. When the mass fraction of Y was 0.35wt%, the alloy had the best mechanical properties with tensile strength and yield strength at room and high temperature (200 ℃) were 207.65, 159.77 MPa and 192.52, 134.83 MPa, respectively.