Photocatalytic H2 evolution technology has sparked intense attention as a potential solution to the energy problem and environmental pollution. Cu2O has emerged as a promising photocatalyst for H2 production due to its narrow bandgap, suitable conduction band (CB) position, and easy synthesis. However, overcoming the disadvantage of photocorrosion, which limits the use of Cu2O in hydrogen production, remains a problem. In this work, g-C3N4 nanosheets were introduced as a support and hole acceptor for Cu2O. Experimental results and density functional theory calculation confirmed the creation of heterojunctions at the interface between Cu2O and g-C3N4 nanosheets, which allows the transfer and separation of photogenerated carriers, resulting in an increased photocatalytic H2 production rate (1796.8 μmol/g/h) and outstanding stability. In addition, the composite photocatalyst exhibits an enhanced tetracycline hydrochloride (TC) degradation activity. A “Z-scheme” photocatalysis mechanism involved in the improvement of the composite photocatalyst’s capacity was clearly illustrated. This study reveals that two-dimensional materials with rich functional groups can modify Cu2O for effective photocorrosion inhibition and enhanced photocatalytic activities.