In this paper,a mathematical model is established to describe the interaction and coupling of acoustic,flow and thermal fields in the semi- continuous casting of aluminum alloy with ultrasonic treatment (UST),and thedistribution of acoustic,flow and thermal fields is numerically simulated with Fluent.The results show that ultra-sonic plays an important role in the distribution of flow and thermal fields; and that,at a fixed ultrasonic output am-plitude,a 20kHz UST for aluminum melt may result in ideal flow and thermal fields that speed up the heat andmass transfer as well as promote the transition of columnar grain to equiaxed grain.Moreover,it is found from mi-crostructure test that,after the UST,the microstructure of casting ingot is well refined,and the average sizes ofgrains in length and width directions reduce by 133μm and 32μm,respectively.The mechanism of grain refinementof large- scale aluminum alloy ingot caused by UST is finally analyzed based on the simulation results.