Ocean contains huge biomass resources. Actively developing marine biomass resources has great significance to human development. In this work, a kind of seaweed, sargassum, is used to prepare chars by pyrolysis. The sargassum chars are modified by NH4Br reagents using a simple impregnation method, and are applied to capture Hg-0 from flue gas. The physicochemical properties of the samples are investigated using scanning electron microscopy (SEM), proximate and ultimate analysis, X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET), respectively. The effects of pyrolysis temperature, reaction temperatures, loading values and the components of flue gas such as O-2, water vapor, NO and SO2 on mercury removal efficiency are studied. Mercury removal mechanism, kinetics and thermodynamics are also investigated. The results show that increasing pyrolysis temperature, reaction temperature, loading value, O-2 concentration and NO concentration enhances mercury removal. Low concentrations of SO2 and H2O are beneficial to mercury removal, and high concentrations of SO2 and H2O inhibit mercury removal. C-Br and C = O covalent groups are the major chemisorption sites for Hg-0 removal. Finally, the kinetic and thermodynamic parameters of mercury removal are also obtained.