Semi-solid Al2Y/AZ91 magnesium matrix composites slurry was prepared by ultrasonic vibration, and the apparent viscosity of semi-solid slurry was measured as well. According to the experimental results, with the increase the of volume fraction of Al2Y the apparent viscosity of semi-solid Al2Y/AZ91 magnesium matrix composites slurry increases, the relationship between apparent viscosity and solid fraction varies according to the exponential law, under the same experimental conditions, the apparent viscosity decreases with the increase of ultrasonic power. Based on the experimental data fitting, the rheological model of semi-solid Al2Y/AZ91 magnesium matrix composites prepared by ultrasonic vibration was established.