In order to solve the problem of coal flue gas mercury monitoring, an online monitoring system for flue gas mercury is established based on transverse Zeeman effect. When a mercury lamp is placed in magnetic field with magnetic induction intensity of 1.5 T, the resonant absorption line at 253.65 nm is split into π and σ± linearly polarized light because of Zeeman Effect. Mercury atom can absorb π linearly polarized light but can not absorb σ± linearly polarized light, so the system can realize the accurate correction for interference background including the interference gases, particulate matter and so on. The low detection limit of this system is 66.3 ng/m3. The mercury detection is not affected when the volume fraction of NO2 gas is not larger than 5.09×10-4 and the volume fraction of SO2 gas is not larger than 1.83×10-4. The monitoring for flue gas mercury in coal-fired power plant lasts 8 days. The average mass concentration of 8.3 μg/m3 and maximum mass concentration of 19.4 μg/m3 of flue gas mercury are obtained, and they are lower than that in national standard. The experimental results show that the flue gas mercury online monitoring system can realize accurate correction for background interference which is caused by interference gases and particulate matter, and realize accurate measurement for flue gas mercury. The system is proper to be used for long-term online monitoring of flue gas mercury.