The molecular dynamics method was used to simulate thermodynamic properties of two binary alloys: Al-75 Si-25, Al-85 Si-15..They were calculated of the energy functions, including cohesive energy, formation energy Results display, formation energy and excess free energy are all negative values, so Al-Si alloys belong to negative system. The atomic interactions were analyzed in macroscopic and microcosmic views. The calculated formation energy can describe the deviation degree between the actual alloy and the ideal melt quantitatively. Al-Si alloy has excellent casting performance and comprehensive thermodynamic performance. It can be well separated with vacuum distillation, and the noble metal Au will be greatly enriched. Thermodynamic properties of an alloy play a key role in vacuum distillation metallurgy. But due to a lack of experimental thermodynamic data of the Al-Si alloy, it is difficult to calculate the reaction concerned with the Al-Si alloy. Therefore, the computer simulation method becomes effective means. Molecular dynamics (MD) simulation has been widely used in the material and field, many reports about the defect, surface property and liquid structure of pure metals and alloys, have come forth from 1980 s([1-3]). However, MD has rarely been used in metallurgy, because there is less simulation on thermodynamic properties of metals and alloys at home and abroad. As thermodynamic properties play a key role in metallurgy, some energy functions of Al-75 Si-25, Al-85 Si-15 have been calculated and been compared with the available experimental ones over wide temperature range from 458to 1358 K in this paper.