The development of catalysts with high activity and selectivity for electrochemical reduction of CO2 could provide renewable energy sources and improved environmental remediation. However, most current electrocatalysts have large energy barriers that lower their activity and selectivity. Here, we designed a class of electrocatalysts with 3d transition bimetallic active sites embedded into covalent organic frameworks (COFs) and discovered that they were more efficient than noble metals and single-atom electrocatalysts in CO2 reduction reaction to CO. To facilitate rational design and quick screening of desired catalysts, an intrinsic descriptor was identified for the catalysts, which correlates the catalytic activity with the topological, bonding, and electronic structures. Among the catalysts, Fe/Cu-, Ni/Zn-COFs could be excellent electrocatalysts, with the low overpotential (similar to 0.25 V) while suppressing side reactions. The excellent activity and selectivity of the catalysts stems from their unique bimetallic sites that break the constraint of scaling relations, leading to much more adsorption modes and facilitating intermediates to achieve the ideal states. (c) 2020 Elsevier Inc. All rights reserved.