The high temperature reduction (HTR) of flue gas divalent mercury to elemental mercury is the core technology for the mercury continuous emission monitoring system (Hg-CEMS). In this work, the high temperature reduction of HgCl₂ to Hg⁰ was systematically investigated. The results show that HgCl₂ can entirely decompose to Hg⁰ at 800 °C, but Cl, ClO, Cl₂, and HCl can easily oxidize Hg⁰ at low temperature. A new efficient alkaline dechlorination agent for eliminating these Cl species was proposed firstly. The dechlorination agent can efficiently capture the HCl, Cl, and ClO but fail to completely adsorb Cl₂. Consequently, a scheme was created that a pair of dechlorination agents were filled in inlet and outlet of the HTR furnace respectively. It features that the upper layer removes HCl and prevents the generation of Cl₂ because HCl is the precursor of Cl₂, and the lower layer adsorbs the Cl and ClO generated by the HTR process. The feasibility of the method was successfully verified in both a fixed-bed reactor and a Hg-CEMS pretreatment device. DFT calculation indicates that the adsorption of Cl, ClO, and HCl on CaO falls into the category of strong chemisorption while Cl₂ is weakly chemisorbed.