Abstract: The activities in Bi-Pb, Bi-Sn, Cd-Pb, Pb-Sn binary alloy system were calculated by Miedema model, molecular interaction volume model (MIVM), non-random two-liquid (NRTL) equation and thermodynamic modeling software FactSage respectively, and compared with measured value. The experimental results show that the Miedema model, MIVM model, NRTL model, Wilson model are not applicable to the above-mentioned four binary alloy system. But the measured activities has good corresponding with the calculated activities by thermodynamic modeling software FactSage, and the activities are calculated by using FactSage at different temperature. Meanwhile, a thermodynamic model for calculating the mass action concentration of structural units in Bi-Pb binary alloy system based on the atom-molecule coexistence theory has been developed, and the expression of standard Gibbs free energy change of reaction for forming intermetallic molecules as BiPb was put forward.