In this paper, the adsorption performance of modified fly ash was evaluated using a fixed-bed adsorption device. The influence of different inlet concentrations of mercury in simulated flue gas and different concentrations of HBr solution on the mercury absorption process was studied. The results showed that, with the increase of the inlet mercury concentration in the simulated flue gas, the adsorption capacity and adsorption rate of the modified fly ash increased. With the increase of the HBr concentration, the adsorption amount and adsorption rate of mercury in simulated fly ash also increased. It was also found that the adsorption process follows the pseudo-first-order adsorption kinetic model.