Sodium silicate, a new type of CO2 sorbent, has a relatively low cost, but its sorption reactivity is not yet good enough. Alkali carbonate doping is commonly used as an effective means to improve the CO2 uptake reactivity of solid sorbents. In this study, sodium orthosilicate, Na4SiO4, was synthesized and mixed with 5, 10, and 20 mol% of Li2CO3–Na2CO3 or Li2CO3–Na2CO3–K2CO3 as CO2 sorbents. The promotion of alkali carbonates on Na4SiO4 in CO2 capture was characterized using thermal analyses in an 80 vol% CO2–20 vol% N2 atmosphere. The phase evolution and structural transformations during CO2 capture were characterized by in situ XRD and Raman, and the results showed that the intermediate pyrocarbonate, C2O52−, which emerged from alkali carbonates, enhanced the CO2 capture of Na4SiO4 to form Na2CO3 and Na2SiO3 from 100 °C. Isothermal analyses showed that 10 mol% of Li2CO3–Na2CO3 was the optimal additive for Na4SiO4 to attain better CO2 uptake performance. The alkali carbonates were effective in reducing the activation energy for both chemisorption and bulk diffusion, improving the cycle stability of Na4SiO4.