Photodetectors are crucial optoelectronic devices that facilitate the conversion of optical signals into electrical responses. They can be broadly categorized into two types based on their operational mode: externally powered and self-powered devices. Cobalt(Ⅱ, Ⅲ) oxide (Co₃O₄) has garnered significant attention as a promising semiconductor material for optoelectronic applications owing to its non-toxic nature, abundance on Earth, and dual optical band gaps. Graphene, with its exceptional optical, electrical, and mechanical properties, along with a large specific surface area, presents itself as an ideal candidate for use in doping materials. Bismuth vanadate (BiVO₄), an N-type semiconductor, is recognized for its excellent stability under harsh conditions and has attracted widespread research interest in areas such as photoelectrochemical (PEC) water splitting, energy storage, dye degradation, and photocatalysis, due to its non-toxic, abundant, and cost-effective characteristics. By doping graphene to modify the surface of Co₃O₄ and constructing a heterojunction via the PN semiconductor principle, a novel approach for self-powered photodetectors is proposed. In this study, Co₃O₄ thin films and Co₃O₄/graphene(G) composite films, with varying graphene molar ratios, were successfully fabricated on fluorine-doped tin oxide (FTO) conductive glass using a hydrothermal method. The optimal graphene doping ratio for Co₃O₄/G films was determined, and BiVO₄ was subsequently spin-coated onto the surface of the Co₃O₄/G films, resulting in the fabrication of (Co₃O₄/G)@BiVO₄ composite thin-film-based photodetectors. Photodetector performance was evaluated using an electrochemical workstation (CHI760E) and a xenon lamp (CEL-S500) to simulate sunlight and measure the photogenerated current. Raman spectroscopy was employed to confirm the presence of graphene and evaluate its defect levels. The microstructure of the samples was analyzed using Field-Emission Scanning Electron Microscopy (FESEM) and transmission electron microscopy (TEM). The phase and chemical bonding states of the samples were investigated by X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS), while optical absorption properties were characterized using UV-Vis spectrophotometry (UV-3600). The results indicated that the Co₃O₄/G film with a graphene doping ratio of 1∶2 exhibited the most stable photogenerated current, with minimal decay, and its photocurrent was 10.8 times greater than that of pure Co₃O₄ films. Furthermore, the morphology of the (Co₃O₄/G) film transitioned from a uniform, compact grass-like structure to a graphene-like network. BiVO₄, prepared by spin-coating, formed a block-like coating on the Co₃O₄ nanorods. The (Co₃O₄/G)@BiVO₄ composite film exhibited enhanced photocurrent and light absorption properties, with a photocurrent 6.3 times higher than that of Co₃O₄/G films. The device demonstrated a responsivity of 2.52 mA·W^-1 and a detectivity of 2.693×10¹² Jones. This composite film structure can provide an ideal research idea for the preparation of simple, non-toxic and harmless, and miniaturized high-performance self-powered photodetectors.