A series of nanosheet MgO-based sorbents promoted by mixed-alkali-metal nitrate and carbonate were prepared by a simple precipitation-deposition method and applied to CO2 capture. The structural properties of these sorbents were characterized by various techniques, and their CO2 capture performance was evaluated using a thermogravimetric analyzer. Compared to the sorbent derived from commercial MgO, the nanosheet MgO-based sorbent had faster and higher CO2 uptake, because of its thin-sheet structure. Among various alkali-metal-salt-promoted nanosheet MgO, the sorbents promoted with mixed-alkali-metal nitrates (LiNO3 and KNO3) and carbonates (Na2CO3 and K2CO3) exhibited a high total CO2 uptake at a respectable rate. In particular, the sorbent with a MgO content of 73 wt % and a nitrate/carbonate molar ratio of 2 possessed the highest total CO2 uptake and the best cyclic stability, with a total uptake of 0.52 g(CO2)/g(sorbent) and MgO conversion of 65% after 20 cycles (60 min of absorption in 40% CO2/60% N-2 at 350 C, 15 min of regeneration in 100% N-2 at 400 C). Based on the structure-activity relationship of the sorbents, a possible mechanism consisting of three stages was presented for the CO2 absorption process, in which the dissolution of CO2 and carbonation in the molten nitrate played an important role.