The presence of gas species other than the target adsorbate in practical conditions causes complications in implementing a sorption-based process because of synergistic or detrimental effects on the sorption mechanism. MgO-based sorbents have attracted massive attention for the pre-combustion CO2 capture, where H2O and CO coexist in emission gas. Although H2O has been reported to greatly enhance CO2 sorption kinetics, CO2 sorption behavior and mechanism under a large quantity of H2O as in the practical condition remain unclear. Furthermore, CO2 sorption behavior in the presence of CO has not been adequately examined. Here, the dynamic CO2 sorption behavior of MgO-based sorbent was understood by coupling the breakthrough experiment under practically relevant conditions with in situ DRIFTS results. At elevated pressures, the sorption performance was considerably enhanced, whereas the influence by temperatures was insignificant. In addition, H2O enhanced CO2 sorption kinetics and initial CO2 sorption on MgO-based sorbent, whereas CO hindered them. In situ DRIFTS analyses for the mechanism study revealed that OH groups generated from H2O dissociation created highly basic sites that promoted the formation of monodentate carbonate, thereby enhancing the initial CO2 sorption. In contrast, CO hampered CO2 sorption through two mechanisms: (1) competitive sorption between CO2 generated from the water-gas shift reaction and CO2 in the feed; (2) interference of the promoted sorption by the molten nitrate salts. This study demonstrated that the practical condition of the pre-combustion CO2 capture significantly alters the CO2 sorption behavior of MgO, thereby providing insights for the ultimate implementation of MgO-based sorbents.