The temperature dependence of the viscosity of liquid Al-12wt.%Sn-4wt.%Si was studied with a high-temperature viscosity apparatus. Anomalous changes of viscosity of the melt were found at 1,103 K and 968K in the cooling process, which indicates anomalous structural changes of the melt. It is calculated that the anomalous structural change is associated with an abrupt decrease of atomic clusters’ size and activation energy in the melt.According to the temperature of the anomalous structural changes, melt heat treatment process (quenching from superheat to pouring temperature)was performed on Al-12wt.%Sn-4wt.%Si melt prior to pouring, aimed to keep the small atomic clusters from higher temperature to lower pouring temperature. The results suggest that relatively small atomic clusters at the pouring temperature in the melt could generate a deep under-cooling of nucleation in the subsequent solidification process, and refine the as-cast structure. After being quenched from superheating to pouring temperature, the relatively small atomic clusters, especially the Si-Si clusters in the melt will grow to equilibrium state (relatively big atomic clusters) with holding time, resulting in the prominent coarsening of the Si morphology in the as-cast structure.