The effect of different ultrasonic power, duration and holding time on AZ80 magnesium alloy melt purification by ultrasonic field was studied. The results indicate that the ultrasonic treatment can accelerate separation of inclusions in the melt and attain melt purification. When the ultrasonic power is 80 W at 650 oC, ultrasonic duration 60 s, and holding time 100 s, the melt has the best purification effect. The more inclusions the ingot has, the lower conductivity is, so the conductivity can characterize the level of purification.