Abstract: Slurry sample was collected from a 300 MW ultra-low emissions coal-fired power unit. The migration and transformation behaviors of mercury in the sample were investigated, and the effect of additive on the stability of mercury in solid gypsum was explored by considering the thermal release behavior and environmental risk. The results show that gaseous Hg⁰ is increased with the increase of slurry temperature, while Hg is increased in both gas phase and gypsum with the increase of slurry pH. The concentration of Cl⁻ or \rmSO^2-_4 increases in slurry could inhibit the reduction of Hg²⁺ to Hg⁰ and increase Hg proportion in gypsum. However, the increase of \rmSO^2-_3 concentration is beneficial to the Hg enriched in gypsum and a part of Hg²⁺ reduced to Hg⁰. When Na₂S, EDTA-2Na or DTCR-4 is added, Hg²⁺ is turned into HgS, Hg(EDTA)₂ or −Hg-DTCR −_n, respectively among which more than 75% Hg is transferred to gypsum and Hg²⁺ is inhibited to reduce into Hg⁰. The thermal stability of Hg in gypsum can be ordered as Gypsum + EDTA-2Na < Gypsum + DTCR-4 < Gypsum + Na₂S < Gypsum due to the stability difference among Hg(EDTA)₂, −Hg-DTCR −_n and HgS(black). By using TCLP, SPLP and MEP, the chemical stability of Hg in gypsum can be ordered as Gypsum < Gypsum + Na₂S < Gypsum + EDTA-2Na < Gypsum + DTCR-4 due to the concentration difference of water soluble mercury, acid soluble mercury and oxidizable mercury among gypsums.