Based on the Miedema model for calculating the formation heat of binary alloy system, activity of each binary system in SiAlBa alloy system at 1873K was calculated combining the relative thermodynamics data. The results show that the activity calculation results of SiAl alloy are in good agreement with the experimental data using the Miedema model. The activity of SiBa and AlBa binary system are also calculated to provide the theory for the alloy design and production of barium bearing alloys.