In cluster #1, the terminology "metal-organic frameworks" appears most frequently. Metal-organic frameworks (MOFs) are new porous materials composed of metal clusters and organic ligands with high specific surface areas and controllable pores. Organic ligands in MOFs can be used as light-harvesting units, and metal clusters play the role of semiconductor quantum dots. In addition, the optical absorption characteristics of MOFs can be regulated by combining different organic ligands and metal clusters. Therefore, MOFs have been regarded as potential photocatalysts. POM-based MOFs single-crystal materials mainly include three structures, as shown in Scheme 1(a). The first type is POM encapsulated in MOFs as guest molecules called "polyoxometalate@Metal-Organic Framework (POM@MOF)". Scheme 1(b) illustrates the synthetic strategy for constructing POM@MOF from MOFs and POMs precursors. The second type is the framework structure formed by POM directly connected with organic ligands. The third type is the framework structure of POM connected with metal and organic ligands. The latter two types of structures are defined as "polyoxometalate-based metal-organic frameworks (POMOFs)". POM-based MOFs can overcome the shortcomings of POMs, such as weak visible spectral absorption and unstable structure in the aqueous solution. In recent years, many POM-based MOFs have been reported for various types of photocatalytic reactions. In particular, water splitting reaction, CO2RR and water pollutant degradation. In 2015, Zhang et al. encapsulated [P2W18O62] 6-in the [Ru(bpy)3] 2+ -based UiO MOF by charge-assisted self-assembly method to achieve visiblelight-driven hydrogen evolution reaction (HER) for the first time (Scheme 1 (c)). Reproduced with permission. Copyright 2021, Royal Society of Chemistry. (b) Routes for the synthesis of POM@MOF from precursors of POMs and MOFs. Reproduced with permission. Copyright 2020, American Chemical Society. (c) The framework and photocatalytic diagram of POM@UiO. Reproduced with permission. Copyright 2015, American Chemical Society. (d) The framework and photocatalytic diagram of POM@MOF-545. Reproduced with permission. Copyright 2018, American Chemical Society. (e) The framework and photocatalytic diagram of (PW12, Cp*Rh)@UiO-67. Reproduced with permission. Copyright 2020, American Chemical Society. (f) The frameworks of POMOF 1 and POMOF 2. Reproduced with permission. Copyright 2018, American Chemical Society.