H2O2 is produced industrially by the anthraquinone method, in which energy consumption is high because of its multistep hydrogenation and oxidation reactions. In this work, hollow copper-doped graphitic carbon nitride (gC3N4) microspheres with outstanding photocatalytic H2O2 production ability are prepared. The characterization results demonstrate that Cu+ is not present as the oxide but instead inserts at the interstitial position through coordinative Cu(I)-N bonds. These Cu(I)-N active sites can act as chemical adsorption sites to activate molecular O-2. Moreover, as an "electron transfer bridge", Cu(I)-N active sites promote electron transfer from the catalyst to the adsorbed O-2 molecules. The as-prepared copper-doped g-C3N4 displays much higher H2O2 equilibrium concentration and formation rate than neat g-C3N4 prepared by calcination does, as well as excellent structural stability. Density functional theory simulations show that Cu(I)-N active sites can adsorb the O-2 molecules with high adsorption energy and elongate the O = O bond. Charge density difference results confirm the electron transfer from the Cu+ doping sites to the O-2 molecules. The Mulliken charge is -0.51 when the O-2 adsorbed on Cu+ doping sites. This electron-rich environment is beneficial to the H+ attack to form H2O2.