The effect of high-intensity ultrasonic treatment (which can generate cavitation) on the microstructure of aluminum (Al) ingots has been widely studied. In this work, a stainless steel thin-walled sleeve was placed in a crucible to separate the 2219 Al melt into two parts: inside and outside the sleeve. By applying the ultrasound in the melt inside the sleeve, a low-intensity ultrasonic field (where the acoustic pressure is lower than the cavitation threshold) was introduced into the melt outside the sleeve. The influence of low-intensity ultrasound on the temperature gradient of the pasty melt as well as the refinement efficiency was explored. The result shows that low-intensity ultrasonic can induce high-frequency forced vibration to the grains and reduce the solid–liquid interfacial thermal resistance. In addition, by comparing the microstructure inside and outside the sleeve, it was found that there was no significant difference in the size and morphology of the grains and eutectic phase. This indicates that the cavitation effect is not necessary for grain refinement, and low-intensity ultrasonic can also produce a significant microstructure improvement on Al alloys.