As the main flocculant of flue gas desulfurization wastewater treatment, polyaluminium chloride has the problems of large dosage and high cost in application.By observing the clarity of sewage and determining the concentration of pollutants in sewage, the flocculation effect of polyaluminium chloride, polyacrylamide with different molecular weight and their combination on flue gas desulfurization sewage from catalytic cracking unit is studied by spectrophotometry and chemical titration. The results show that the flocculation of polyaluminium chloride and polyacrylamide was superior to the flocculation of polyacrylamide alone, and the larger the relative molecular mass of polyacrylamide, the better the flocculation effect. At the same time, the effects of flocculant dosage, sewage pH value and stirring time on the flocculation effect were analyzed and optimized. The results showed that 30 mg/L polyaluminium chloride and 1.0 mg/L ultra⁃high molecular weight polyacrylamide were the best flocculation effect on catalytic cracking flue gas desulfurization wastewater at pH 7~8 and stirring time 4~6 minute. The research results also laid a solid foundation for the cost optimization of sewage treatment in the future.