Achieving both high CO/CO2 selectivity and large CO working capacity in an adsorbent is very challenging. In this work, we have loaded Cu(II) species into large pores of an iron-based metal-organic framework (MOF) with a large surface area and reduced them into Cu(I) species under mild conditions by utilizing Fe(II) sites in the pores. Remarkably, the Cu(I)-incorporated MOF (0.9Cu@MIL-100) exhibits a high CO/CO2 selectivity (29 at 100 kPa) and a large CO working capacity (1.61 mmol/g at 10-100 kPa) simultaneously, which has not been observed for previously reported adsorbent materials. Moreover, 0.9Cu@MIL-100 also presents very high CO/CH4 and CO/N-2 selectivities (87 and 677). Furthermore, breakthrough and cyclic adsorption-desorption experiments confirm that this material can efficiently separate CO/CO2 mixtures under dynamic mixture flow conditions and can be easily regenerated under mild conditions. This study provides a new strategy for developing adsorbents with both high CO/CO2 selectivities and large CO working capacities.