To overcome the drawbacks of high solubility and instability of polyoxometalates (POMs) in aqueous solution and to expand their application in the electrocatalytic reduction of CO2 (ECR), we assemble sandwich-type POMs, K10[(PW9O34)2M4(H2O)2] (M = Mn, Ni, Zn, shortened as P2W18M4), into the hexagonal channel of a porphyrin-based metal–organic framework (MOF) PCN-222 to form P2W18M4@PCN-222 composites. Their ECR behavior displays polyoxoanion-dependent activity. P2W18Mn4@PCN-222 demonstrates a faradaic efficiency of 72.6% for the CO product (FECO), more than four times that of PCN-222 (FECO = 18.1%), and exhibits exceptional electrochemical stability over 36 h. P2W18Ni4@PCN-222 and P2W18Zn4@PCN-222 slightly increase (26.9%) and decrease (3.2%) in FECO, respectively. We combine the results with density functional theory (DFT) calculations to help understand the intrinsic reasons which reveals that the rate-determining step (RDS) reaction energy of P2W18Mn4@PCN-222 and P2W18Ni4@PCN-222 is significantly reduced compared to that of PCN-222. It is different in P2W18Zn4@PCN-222. Frontier molecular orbitals electron distribution results hint at directional electron transfer from P2W18Mn4/P2W18Ni4 to the porphyrin ring active center in PCN-222, promoting the electro-reduction of CO2 activity. By contrast, P2W18Zn4 may accumulate electrons from PCN-222, thus facilitating the hydrogen evolution reaction (HER). This work reveals the critical role of sandwich-type POMs in manipulating the electron transfer pathway during the electrocatalytic process. Our findings would broaden the scope of POM applications in electrochemical carbon dioxide reduction. [ABSTRACT FROM AUTHOR]