Abstract: Diffused and reactive O atom in process of internal oxidation in Ag-Sn-O system was calculated by density functional perturbation approximation theory (DFPT). The results showed binding energy, enthalpy, heat capacity and Gibbs free energy of the phases in Ag-Sn-O system. O atom was on tetrahedron positions in Ag lattice. The system energy of O atom on the octahedron positions was 288.23 kJ·mol-1 higher than that of O atom on the tetrahedron positions. However, Sn atom was on the replacement positions in Ag lattice. The order of cohesive energies (Ec) in Ag-Sn-O system was Ag6O2>SnO2>Ag2SnO3>SnO>Ag2O. The enthalpy of SnO2 was -591.1 kJ·mol-1, which was in agreement with experiment. SnO, Ag6O2, and Ag2SnO3 were the metastable phases in the normal temperature. Because the bonding ability of Sn—O was higher than that of Ag—O, there was no silver-oxide in Ag-Sn-O system. The heat capacity of Ag2SnO3 was much bigger than those of other compounds, and its Debye temperature was about 500 K. SnO2 was the most stable phase in Ag-Sn-O system, which was showed by population analysis and Gibbs free energy.