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[Display omitted] • A novel alkaline filler is developed for improving the efficiency of high temperature reduction of HgCl 2. • Cl species adsorption behaviors on earth metal oxides are predicted by DFT. • Frontier molecular orbital reveals the orbital energy level difference is intrinsic driving force of binding difference. • The optimal filler is calcium acetate monohydrate as precursor with 50% loading and SiC as carrier. • Field experimental validation demonstrates industrial feasibility of alkaline filler with an error below 7%. The reduction of Hg2+ to Hg0 through high temperature is a crucial technique for mercury monitoring. However, this application encounters two challenges: suboptimal thermal conditions in the reaction zone leading to incomplete decomposition of Hg2+ and the susceptibility of the decomposed Hg0 to re-oxidation by Cl species. This work proposes a novel alkaline filler with alkaline earth metal oxides (AEMOs) as the active component, which enhances the thermal conditions of the reaction while effectively removing Cl species. DFT reveals increased adsorption energies of AEMOs for HCl and Cl with the growing atomic period. Further comparison of the electronic structures and adsorption behaviors between AEMOs and Cl species indicates a similar trend in electron transfer and electron density at the bond critical points. The frontier molecular orbital analysis shows a positive correlation between orbital energy level difference and adsorption energy. Experimental validation of the DFT results, combined with a comparison of costs and de-HCl performance, identifies CaO as the optimal active component. Subsequently, the CaO is loaded onto SiC, and the impact of precursor and loading ratio on the composite performance is investigated through experiments and characterization. The optimal preparation method involves using calcium acetate monohydrate as the precursor with a loading rate of 50 %. Finally, the developed alkaline filler is tested in a Hg-CEMS for industrial application, it shows errors of below 7 % compared to the EPA Method 30B results, validating its industrial feasibility. This work offers a practical solution for Hg2+ high-temperature reduction in industrial settings. [ABSTRACT FROM AUTHOR]