The effects of addition of Y with ultrasonic field assistance on as-cast microstructure of AZ91 alloy were studied. The predominant precipitated phases of AZ91-Y alloy system were worked out to be Al-Y intermetallics by Miedema’s model, and the new precipitated phases were verified to be Al2Y by the result of XRD. The phases and microstructure of the composites were also analyzed. The result shows that when the content of Y in AZ91 alloy exceeds 1.0%, the precipitated phases Al2Y grow up and agglomerate gradually, the grain refinement effect is not obvious. After the AZ91-Y melt are treated by ultrasonic for 2 min at 850 °C, Al2Y particles are refined and distribute uniformly, and the morphology of β-Mg17Al12 changes from continuous to discontinuous or spherical morphology, while the grain and microstructure of the alloy are refined obviously