In CO electroreduction, the critical bottleneck lies in the CO activation which requires high overpotentials. CO activation is related to both the electronic structures of catalysts and those of adsorbates, thus an ideal catalyst should match its electronic structures with those of the adsorbate. Here, we harmonized the electronic structures of the adsorbate and Mn-doped InS nanosheets for efficient CO reduction. The introduction of Mn dopants into InS nanosheets enhanced both the Faradaic efficiency (FE) for carbonaceous products and current density (). At -0.9 V vs RHE, Mn-doped InS nanosheets exhibited a remarkable FE of 92% for carbonaceous product at a high of 20.1 mA cm. Mechanistic studies revealed that Mn doping enabled the harmonic overlaps between the orbitals of O atoms and orbitals of Mn atoms near the conduction band edge of Mn-doped InS nanosheets during the activation of CO. Due to the unique electronic structures of the coadsorbed configurations, Mn-doped InS nanosheets exhibited an energy barrier for CO activation into HCOO* lower than that over pristine InS nanosheets.