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The microstructures of Al-20Si alloy solidified under different pressures (2 GPa and 3 GPa) and different superheat temperatures were investigated. The results demonstrate that the decrease of superheat temperature leads to the changes in morphology of the α-Al phase under high pressure (from dendritic, to cellular, to globular and to planar growth). Furthermore, a new mathematical model was developed to analyze interface stability under the combination effects of pressure and superheat temperature. The interface stability of α-Al phase in Al-20Si alloy was calculated at the pressure range of 2−3 GPa and the superheat temperature range of 0−600 K. The result shows that the transition superheat temperature of α-Al phase planar growth with the pressure of 2 GPa and 3 GPa is 53 K and 111 K, respectively. The interface of α-Al phase can remain stable at higher superheat temperature with the increase of pressure.