Abstract: A multi-field coupling mathematical model was established to analyze the hot-top semi-continuous casting of a large-scale aluminum alloy of 630 mm in diameter with ultrasonic treatment (UST). The finite volume method combined with the self-com-piled function was used to get the distribution of the acoustic field, flow field and temperature field, and the industrial experimental re-search was also performed. The mechanism of ultrasonic treatment on the grain refinement of the large-scale aluminum alloy produced by hot-top semi-continuous casting was analyzed based on the simulation and experiment results. The simulation results show that ultra-sonic markedly influences the macroscopic physic field. With ultrasonic treatment, an upward recirculation zone forms beneath the end face of the radiation rod, the heat and mass transfer is promoted by the strong turbulence, the sump gets shallower and gentler, the ini-tial solidifying point on the ingot surface in the mold moves down, the transition zone becomes narrower, the width of it in the center drops from 342 mm to nearly 120 mm. The experimental results show that with ultrasonic treatment, the grain size becomes smaller and more homogeneous, the average grain size reduces by 103μm, the gap between the maximum and minimum grain size decreases from 135 to 64μm, and the grain boundary of the solidification structure becomes thinner.