Reversible CO2 capture from ambient air by a humidity swing has shown great potential in mitigating the greenhouse effect. In this work, we developed a new humidity-swing absorbent based on PO43-/HPO42-/H2PO4- ions exhibiting superior CO2 absorption capacity and kinetics compared to that of CO32-/HCO3--based absorbent. After ion exchange with PO43- ions, the ion-exchange resin (IER-PO4) containing positive quaternary ammonium groups and movable PO43- ions is able to reversibly capture CO2 from the ambient air by a humidity swing, which triggers the transformation between the PO43- ions and HPO42-/H2PO4- ions in the resin. In a dry environment, PO43- ions in IER-PO4 are hydrolyzed into OH- ions and HPO42- ions, which are further hydrolyzed into H2PO4- ions and OH- ions. Both hydrolysis reactions produce OH- ions for CO, absorption, while the adsorbed CO2 can be released in a humid environment. The results of quantum chemical calculation show that the hydrolysis of the ions is promoted by the reduction of water molecules in the nanoscale hydrated cluster. The adsorption capacity of IER-PO4 during the moisture swing is 80% larger than that of IER-CO3, and the adsorption rate of the IER-PO4 resin at a temperature range of 15-35 degrees C is much higher than that of the IER-CO3 absorbent. A modified pseudo-first-order (MPFO) kinetic is developed, which can describe the experimental results well. The present study sheds light on the design of high performance moisture-swing absorbents with PO43- ions.