Ag/SnO2 composite materials were prepared by reaction synthesis, and the transformation of the alloy intermediate compound to the alloy-rich region, looking for a stable transition state to generate the final phase, was elaborated from the electron-atom level. The results indicate that the activation properties of Sn are higher than Ag, and the participation of oxygen source plays a leading role to the reaction process. The free state O decomposition in the initial reactive mixture is another important way to make the oxidation reaction proceed continually and thoroughly. Finally the complete reaction path was predicted.