The molecular dynamics method was used to simulate thermodynamic properties of three binary alloys: Pb40Au60, Pb80Au20 and Pb90Au10. The energy functions, including excess free energy, cohesive energy and formation energy, were calculated. Formation energy and excess free energy are all negative values, so Pb-Au 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