A thermodynamic calculation and data analysis were carried out for silver tin oxide composite material prepared by reaction synthesis processing using Gibbs Helmholtz equation. The results show that the reactive temperature is below 505 K, the reactive synthesis of silver tin oxide belongs to the substitution reaction of solid-solid. When the reactive temperature is over 505 K, the reactive synthesis of silver tin oxide belongs to the oxidation reaction between oxygen and solid. By the calculation of free energy per unit volume and decomposed oxygen partial pressure of SnO, SnO2 in the rang from 505 K to 1200 K, it can be determined that the final production is Ag phase and SnO2 phase for the oxygen reaction of Ag2O, Ag, Sn. But no SnO phase exists in the material. The analysis of thermodynamics calculation data and oxygen branch-pressure offers an academic foundation for preparing AgSnO2 composite material adopting reaction synthesis processing.