Background noise from SO2, NO2 and other interfering gases in flue gas is corrected by the transverse Zeeman effect. The mercury concentration in flue gas is determined by Zeeman atomic absorption spectrometry. The average concentrations of element Hg (Hg0) in flue gas before and after wet flue gas desulfurization (WFGD) are 0.36 μg·m-3 and 11.08 μg·m-3. The concentration of Hg0 in flue gas increases significantly after desulfurization system. The monitoring results show that about 99% of SO2 in the flue gas is absorbed by the desulfurization slurry, and produces enough sulphite which reacts chemically with Hg2+ to release Hg0. The change of pH value in the desulfurization slurry also accelerates reduction reaction and release of Hg0. The use of WFGD system to remove mercury can lead to the increase of element mercury concentration. The monitoring results of element mercury have certain correlations with concentrations of other components in the flue gas, which is consistent with the theoretical analysis. The monitoring results show that transverse Zeeman atomic absorption spectrometry can effectively overcome interference caused by SO2, NOx and other gases. The results confirm the accuracy and feasibility of transverse Zeeman atomic absorption spectrometry applied in the detection of mercury content in flue gas.