Abstract: The effect of different vibration parameters, including ultrasonic power and duration time, on the degassing of molten Mg-Ca alloy was investigated by using ultrasonic field. The results showed that both the ultrasonic power and the duration time have significant effects on the degassing effect. Low ultrasonic power and short or long duration time have weak effect on the degassing. The degassing effect is related with the ultrasonic cavitation, and the optimum degassing effect only occurs in the developing-developed stage of the cavitation. In this experimental condition, when the ultrasonic power and the duration are 150 W and 120 s respectively, there has the optimum degassing effect and the density of ingot reaches the maximum value. The ultrasound refines effectively the solidification structure of Mg-Ca ingot at the same time.