Gibbs’ function of metal oxide from the reaction between elements as well as sub-metal oxides and 1 mol oxygen in Ag/CuO composites by reactive synthesis was calculated thermodynamically; through the calculation, the relation diagram was drawn for Gibbs’ function and temperature. Considering oxygen pressure influencing the reaction greatly, the oxygen pressure line with temperature change was also drawn in the diagram above. Results show that the reactions between Ag and O2, and Cu2O and O2 are reversible, whose temperature for reversible transition are 586.818 K and 667.663 K, respectively. The oxygen pressure is important for decomposition of metal oxides: the lower the oxygen pressure is, the more easily the oxides decompose. The reactive synthesis sintering technology of Ag/CuO composite was determined by thermodynamic analysis.