Covalent organic frameworks (COFs)-based nanoreactors have attracted broad interest in many fields due to their void-confinement effects. However, the inherent drawback of conventional nanoreactors is the lack of internal active sites, which limits their widespread utilization. Herein, we report the construction of hierarchical COF (EB-TFP) nanoreactor with pre-synthesized polyoxometalates (POM, [PV₂W₁₀O₄₀]^5– (PV₂W₁₀)) clusters encapsulated inside of COF (POM@COF). PV₂W₁₀@EB-TFP anchors nucleophilic-group (Br^– ions) and PV₂W₁₀ anion cluster within the COF framework via electrostatic interactions, which not only simplifies the reaction system but also enhances catalytic efficiency. The reaction performance of the PV₂W₁₀@EB-TFP nanoreactor can be tuned to achieve excellent catalytic activity in CO₂ cycloaddition reaction (CCR) for ~ 97.63% conversion and ~ 100% selectivity under visible light irradiation. A mechanistic study based on density functional theory (DFT) calculations and in-situ characterization was also carried out. In summary, we have reported a method for achieving the uniform dispersion of POM single clusters into COF nanoreactor, demonstrating the potential of POM@COF nanoreactor for synergistic photothermal catalytic CO₂ cycloaddition.