Development of semiconductor catalysts is an attractive way to solve energy shortage and pollution problems. In this work, a kind of ternary heterojunction nanocomposite g-C₃N₄/Co₃O₄/Ag₂O was successfully prepared, whose morphology, structure and composition were characterized by TEM, EDS, XRD and XPS analysis. Photocatalytic experiments prove that g-C₃N₄/Co₃O₄/Ag₂O shows good photodegradation efficiency (98.09%) for RhB under visible light. The free radical capture experiment indicates that the hole, superoxide radical and hydroxyl radical all contribute to the photocatalytic process. In addition, it is found that introducing Ag₂O into the composite can efficiently promote the formation of H₂O₂via the reduction of O₂. An electrochemical oxygen reduction reaction (ORR) study further confirms that the value of electron transfer during the ORR is 2.07 for g-C₃N₄/Co₃O₄/Ag₂O, indicating the promoted two-electron O₂ reduction reaction to H₂O₂. Based on the above results and energy band structures of semiconductors, a possible photocatalysis mechanism is proposed.