Abstract: The trend of zero wastewater discharge policy is inevitable for coal-fired plants despite the major technical difficulty in the discharge of wet limestone-gypsum FGD wastewater. To obtain reasonable discharge amount of FGD wastewater, the author took the example of a 350 MW coal-fired unit in the case studies with the source and discharge of chloride ion amount as the starting point. Then benchmarked on the chloride ion balance concentration in the absorber slurry, the material balance of chloride ion from coal input, FGD process water, gypsum output, wastewater discharge, etc. were calculated. On this basis, the effects of the engineering projects on FGD water were explored such as in-depth optimization of desulfurization system and in-depth flue gas treatment, which will provide substantial data support for the feasibility research of zero discharge projects for terminal wastewater treatment.