The synergistic removal of mercury by injecting activated carbon between the selective catalytic reduction (SCR) and fabric filter (FF) as well as some influencing factors were investigated in a pilot-scale 0.3 MWth circulating fluidized bed (CFB) combustion system. The Ontario Hydro Method (OHM) was used to sample mercury in the coal-fired flue gas. The gaseous elemental mercury (Hg-0), gaseous oxidized mercury (Hg2+), particle-bound mercury (Hg-P) in the flue gas after the boiler, SCR, activated carbon injection (ACI) device, FF, and wet flue gas desulfurization (WFGD) were sampled simultaneously, and the flue gas components were monitored online. The results showed that the mercury removal efficiency of the SCR+ACI+FF system (SAFs) and the proportion of Hg-P after ACI increased with the increase of the flue gas temperature. The mercury removal efficiency decreased with incremental SO2 concentration probably due to its suppressive effect on the combination of mercury and halogen and its competitive adsorption with mercury. The higher NO concentration can improve the mercury removal efficiency resulting from He oxidation by NO2, which could counteract its adverse effect on mercury oxidation in SCR, The FF device not only could capture all Hg-P but also had a considerable effect on gaseous mercury. The synergistic mercury removal efficiency of the ACI and air pollution control devices (APCDs) system was more than 97.4%. The presented work could provide useful information and guidance for the optimization of the operational parameters of the ACI+APCDs mercury removal technology.