The application of ultrasonic vibration to the casting process can be realized through mould (die) vibration. However, the resonant vibration of the mould is always accompanied by a non-uniform vibration distribution at different parts, which may induce a complex liquid flow and affect the casting fluidity during the mould filling process. The influence of non-uniform ultrasonic vibration on the fluidity of liquid AlSi9Cu3 alloy was studied by mould vibration with different vibration gradients. It is found that ultrasonic mould vibration can generate two opposite effects on the casting fluidity: the first, ultrasonic cavitation in melt induced by mould vibration promotes the casting fluidity; the second, the non-uniform mould vibration can induce a melt flow toward the weak vibration areas and turbulence there, consequently decreasing the casting fluidity. When the melt flow and turbulence are violent enough to offset the promoting effect of cavitation on fluidity, the ultrasonic vibration will finally induce a resultant decrease of casting fluidity. The decreasing effect is proportional to the vibration gradient.