Through the simulation experimentation, the Hg adsorption characteristics of fly ash in flue gas were studied carefully in laboratory. From the experiment results it was found that Hg adsorption capacity of different porous medium in fly ash was different obviously. Unburned carbon had higher Hg adsorption capacity. The surface-area of BET sorbent had positive correlation with Hg adsorption capacity. Hg concentration in blend gas had non-linearity relationship with Hg adsorption capacity. Hg adsorption capacity of 5 x 10(-7) Hg concentration was only 1/2 compared to that of 5 x 10(-6). Adsorption temperature had negative correlation with it. C=O in unburnt carbon surface was beneficial to oxidation and chemic adsorption of Hg. It was obtained that the porous structure and huge surface-area of the unburned carbon was benefit to the Hg adsorption in flue gas.