The magnesium oxide (MgO) sorbents doped with different NaNO3/NaNO2 molar ratios were prepared to improve their stability over multiple cycles. Regulating the NaNO3/NaNO2 molar ratio effectively improved the CO2 adsorption performance of the sorbent. The MgO sorbent with the NaNO3/NaNO2 molar ratio of 0.8 exhibited outstanding performance and stability. Characterization and density functional theory calculations confirmed the positive effects of the appropriate NaNO3 amount in improving the adsorption activity, whereas the suitable NaNO2 amount slowed the sintering step and improved stability. Notably, the molten salt remained stable during cycling and did not decompose even at high regeneration temperatures, guaranteeing the cyclic stability of the sorbent. Further characterization showed that the CO2 sorption drop in the first few cycles was ascribed to the decline in the active surface owing to the aggregation of MgO particles, incomplete regeneration of sorbents, and segregation of molten salt on the sorbent surface.