A renovation project of flue gas off-whitening technology for two 40 t/h coal-fired industrial boilers in a heating center was selected as an example. The technical principle and development status of flue gas off-whitening technology were introduced. The operation effect of flue gas off-whitening system with "spray condensation + flue gas-steam heat exchange" was analyzed. The results showed that the condensing temperature of flue gas was 30~42 ℃, and the humidity of flue gas was 4.5%~7%. The outlet flue gas temperature was higher than 80 ℃. In addition, the dust concentration in flue gas was reduced from 10 mg/Nm3 to 4 mg/Nm3, and the collaborative dust removal efficiency was 60%. The average concentration of SO2 in flue gas was 19.4 mg/Nm3, and the synergistic desulfurization efficiency was 35%. The system operates stably,and the flue gas emission meets the standard for ultra-low emission, which requires the concentration of dust ≤10 mg/Nm3, SO2≤35 mg/Nm3, and NOX≤50 mg/Nm3. It also meets the design requirements of flue gas off-whitening technology.