In recent years, with the increase in agricultural activities, point-source and non-point-source pollution during the process have led to excessive levels of nutrients such as nitrogen and phosphorus in small rural water bodies, causing frequent cyanobacterial blooms. Sodium percarbonate, as a new environmentally friendly oxidant, has gained increasing attention for its application in controlling cyanobacterial blooms. However, existing studies lack research on optimizing its dosage from the perspective of cost-effectiveness. To better guide practical applications, this study simulated severe cyanobacterial bloom conditions and conducted comparative experiments on the effectiveness of sodium percarbonate in treating cyanobacterial blooms at eight different dosages: 0, 2, 3, 4, 5, 6, 7, and 8 mg/L. The results showed that, for severe cyanobacterial blooms, considering treatment efficacy, cost, and ecosystem safety, 6 mg/L was the optimal choice. At this dosage, after 48 h of treatment, the algal cell density decreased from an initial 60 cells/L to 1.5 cells/L, achieving a removal rate of 97.5%. The bloom severity was reduced from severe to no visible bloom. These findings provide a reference for determining the optimal dosage of sodium percarbonate for practical applications in treating severe cyanobacterial blooms.