The effect of NaNO3 on the activation of MgCO3-CaCO3 double salts to form a MgO-CaCO3 absorbent and the cyclic CO2 absorption ability of the obtained absorbent have been investigated to analyze the affecting mechanism of NaNO3. The MgCO3-CaCO3 double salts were prepared by two methods: coprecipitation of Mg(NO3)(2), Ca(NO3)(2), and Na2CO3 and modification of dolomite by NaNO3. The results indicate that NaNO3 has an obvious effect on lowering the activation temperature of the MgCO3-CaCO3 double salts, and NaNO3 is the key component in facilitating the CO2 absorption by the MgO-CaCO3 absorbent. The affecting mechanism has also been analyzed using X-ray diffraction characterization, particle diameter distribution analysis, Brunauer-Emmett-Teller analysis, and the scanning electron microscopy characterization of the absorbents. The results indicate that the molten NaNO3 facilitates the decomposition of MgCO3 in MgCO3-CaCO3 double salts, affects the crystal lattice of the activated absorbents, and favors the CO2 absorption by improving the ion diffusion in the absorbent.