In order to solve the technical problems of simultaneous desulfurization and denitrification of flue gas at ultra-low temperatures (<120℃) by dry process, this work prepared several calcium-based composite absorbents by coupling different strong oxidants with lime of high specific surface area, which can realize the efficient simultaneous removal of sulfur oxides (SOx) and nitrogen oxides (NOx) from flue gas at ultra-low temperature (<120℃). The results of desulfurization and denitrification performance evaluation of calcium-based composite absorbers with different oxidant types and loadings showed that the addition of strong oxidants not only improved the desulfurization performance of Ca(OH)2 but also achieved simultaneous denitrification with Ca(OH)2. The type and amount of oxidant had a significant influence on the simultaneous desulfurization and denitrification of flue gas, and the appropriate moisture and oxygen contents of flue gas were beneficial to improve the desulfurization and denitrification efficiency. According to the characterization results of crystal structure, thermal decomposition, microstructure, and specific surface area of calcium-based composite absorbers before and after the flue gas purification reaction, it can be found that SO2 and NO are first oxidized by strong oxidants, and then react with hydrated lime to generate corresponding sulfates and nitrates, improving the desulfurization and denitrification efficiency. This study is expected to provide a new purification material and process for the upgrading of the intrinsic engineering process or purification of small industrial boilers to achieve ultra-low emission requirements.