Atomic-level structural precision and dynamic control of metal clusters during catalytic processes remain great challenges. Herein, Cu-W₄₈/CNNS single cluster catalyst was fabricated by dispersing wheel-shaped [(Cu(H₂O))₂[Cu₄(OH)₄(H₂O)₈]₂P₈W₄₈O₁₈₄]^28- (Cu-W₄₈) polyoxometalate (POM) onto carbon nitride nanosheets (CNNS). The tetranuclear copper cluster [Cu₄(OH)₄(H₂O)₈] (Cu₄) consists of two binuclear copper units [Cu₂(OH)₂(H₂O)₄] (Cu₂). Encapsulated within the POM cavity, the Cu₄ cluster demonstrates exceptional catalytic performance toward molecular oxygen activation, achieving highly selective monooxygenation of cyclohexene to epoxycyclohexane with unprecedented efficiency (96.5% yield and 97.6% selectivity). Notably, the single-cluster catalyst exhibits remarkable performance across diverse monooxygenation reactions, including epoxidation, aromatic hydrocarbon oxidation and thiooxidation. This work provides new insights into the rational design of atomic-precision multinuclear metal cluster catalysts by precisely integrating transition-metal-oxo clusters into the cavities of monodispersed POMs.