To upgrade a wet flue gas desulfurization (FGD) wastewater treatment process in a typical thermal power plant (TPP) in Hunan province, China, a new concept for reusing polyaluminum chloride (PAC)-based water treatment plant sludge (WTPS) as a coagulant is proposed. Results show that, for an optimal WTPS dosage of 1,000 mg/L, the corresponding removal capacities for suspended solids (SS) and chemical oxygen demand (COD) from the practical FGD wastewater were 58.3% and 40.3%, respectively. Through an advanced treatment with a dosage of 50 mg/L of PAC, pH of 9, and stirring of 150 r/min for 60 s and 50 r/min for 15 min, the total removal efficiencies of SS, COD, and total cadmium (Cd) from the FGD wastewater were 93.7%, 88.8%, and 84.6%, respectively. Therefore, a new modification process (that involves mixing with WTPS - slag cleaner - neutralization - coagulation - sedimentation) was proposed. The proposed process is economically superior, and the average cost for the FGD wastewater treatment was only 1.08 USD/t. This could provide a cost-effective alternative process for upgrading FGD wastewater treatment facilities of TPPs.