NaNO3 modified MgO sorbents have a large theoretical CO2 capture capacity, but their practical capture performance at medium temperatures (350-400 degrees C) is poor for the high thermodynamic equilibrium partial pressure of CO2 at these temperatures. The thermodynamic limitations for NaNO3 modified MgO sorbents can be relaxed by adding alkali or alkaline earth carbonates, and the effective carbonation temperature can be shifted to the higher temperatures (350-400 degrees C) to satisfy the requirement of fast kinetic reactions. In this paper, the CO2 absorption characteristics of NaNO3 modified MgO with or without CaCO3 are compared experimentally to verify the promoting effects of CaCO3. Based on the CaCO3 promoting theory, the methods of stage heating during carbonation and partial decomposition during regeneration were adopted to improve the CO2 absorption ability of NaNO3 modified MgO sorbents at 350 degrees C. Combined with the composition of fuel gas, effects of increasing CO2 partial pressure and H2O were tested, and they both accelerated the carbonation rates of the sorbents, which forecasted the application potential of NaNO3 modified MgO sorbents.