CuO was synthesized by thermal decomposition of Cu(NO3)2·3H2O at various temperatures and characterized by powder X-ray diffractometry (XRD) as well as scanning electron microscopy (SEM). The effects of calcination temperature, category of sacrificial reagent, initial sacrificial reagent concentration, and Ag loading content on the photocatalytic activity of the as-obtained CuO sample were investigated. The results show that the as-obtained CuO exhibits high activity for photocatalysis of H2 evolution reaction (HER) in oxalic acid solution under simulated sunlight irradiation. The highest photocatalytic activity of the as-obtained CuO was achieved at the calcination temperature of 1 000 °C, and oxalic acid was used as the sacrificial reagent with the concentration 0.05 mol/L. H2 evolution rate is as high as 2.98 mmol/(h·g) with 2% (mass fraction) loaded Ag. The possible photocatalytic reaction mechanism on the CuO photocatalyst for HER in oxalic acid solution was also discussed.