The product Hg²⁺ was prone to complexation during the process of wet oxidation to remove Hg⁰ from flue gas. The complexation product may change the oxidation mechanism to form a new reaction mechanism. This phenomenon has not been studied. The effects of ligand cation, ligand concentration, reaction pH, reaction temperature and molar ratio on the complexation reaction of Hg²⁺ and the complexation mechanism in the oxidation removal of Hg⁰ in flue gas were studied. The results showed that MgCl₂, KCl, NiCl₂ and BaCl₂ could be complexed with HgCl₂ but neither CuCl₂ or SnCl₄. The amount of complex formation increased first with ligand concentration increasing and then tended to be equilibrium. The acidic reaction environment was beneficial to the complexation reaction and not affected by the pH change. However, the complexation reaction could not occur in the alkaline reaction environment. The reaction temperature did not affect the complexation reaction. The absorbance of all the complexes produced was basically the same, this value was (4.20 ±0.03) A. The mercury complexation reaction had a response interval to the ligand concentration. When the ligand concentration was less than the lower limit of the interval, complex could not be formed. When the ligand concentration was more than the upper limit of the interval, the complex yield did not change with the ligand concentration. When the mercury atom sp orbit was recombined, it was difficult for the 6d empty orbit to participate in the hybridization to form a high-spin outer rail type complex. The cumulative stability constant β₄ was 10^15.07. The complex[HgCl₄]^2- had the ability to chelate Hg (aq) and O₂ (aq), then react with ClO^- and Hg₂Cl₂. A new oxidation-complexation oxidation reaction mechanism was formed.