Abstract: Based on Miedema for calculating the formation heat of binary alloy system, the activity of each and every binary system in the Si-Ca-Ba alloy system was calculated at 1623 K combining with related thermodynamic data. The calculation results showed that although the activity of Si-Ca alloy confirms fairly with measured data, the activity curves of Si-Ca and Si-Ba alloy melts show relatively great negative deviation from the idea melt. It means that there is a strong interaction between Ca and Si atoms and between Ba and Si atoms. In comparison, the Ba-Ca melt comes nearer to the idea melt in the whole range of composition, and the interaction between Ca and Ba atoms is weaker.