The in-flight mercury removal performance of ammonium bromide impregnated activated Carbon (NH4Br-AC) was evaluated in an entrained flow reactor (EFR) under simulated flue gas. The factors that affect in-flight Mercury removal efficiency were explored. The optimum operating parameters were selected to be verified in the EFR under real flue gas, which was derived from the anthracite combustion in a 6 kW circulating fluidized bed (CFB) combustor. The coeffect of NH4Br-AC injection on SO2 and NO emission was also investigated. The results show that the in-flight mercury removal rate of raw activated carbon (R-AC) is significantly improved by the NH4Br modification. Greater sorbent feed rate, longer sorbent residence time, and smaller sorbent particle size are beneficial for improving the in-flight mercury removal, rate. In the anthracite combustion flue gas, with the increase of sorbent residence time from 0.59 to 1.79 s, the in-flight mercury removal rate of NH4Br-AC increases from 70.7% to 90.5%. Although the physisorption strengths of SO2 and NO are greater than that of gad-phase mercury, the increase of the Br group on the NH4Br-AC surface improves the mercury adsorption affinity. The reduction rates of SO2 and NO reach 30.6% and 38%, respectively, but the SO3 concentration in the flue gas increases 116% compared to the original emission concentration. The reduction of SO2 and NO in the flue gas is attributed to the chemisorption on the NH4Br-AC surface and the oxidation by the injected O-2 existing in the sorbent carrier gas, which promotes more SO3 and NO2 generation in flue gas.