Abstract: The initial rainwater storage tank is a critical facility for intercepting highly polluted runoff during the early stages of rainfall and mitigating urban water environmental pollution. However, due to constraints such as cost, maintenance conditions, and technical limitations, it is often difficult to obtain online water quality monitoring data to guide the real-time operation of rainwater storage tanks. Existing approaches based on total runoff volume control are insufficiently responsive to the temporal variability of rainwater pollutants, which may result in ineffective strategies such as “storing clean water while discharging polluted water” or “resource waste”, ultimately limiting the effectiveness of storage tanks. To address this issue, this study proposed a control strategy based on the cumulative pollutant load ratio. By analyzing monitoring and simulation data from typical rainfall events, the study investigated the pollutant wash-off effect during the early stages of rainfall and identified the critical periods for inflow collection necessary to meet pollutant load reduction targets. Based on these analyses, the operational strategies for controlling rainwater storage tank inflow and overflow gate discharges were optimized. The proposed method was applied to the No. 7 initial rainwater storage tank located in the Lianghe area of Jiujiang City. The results indicate that: (1) A significant first-flush effect exists in the service area of the No. 7 storage tank in Jiujiang, with approximately 80% of pollutants being transported by the first 50% of the runoff volume. (2) Identifying the critical inflow collection period allows the pollutant collection targets to be met while significantly reducing the operational and management costs of the storage tank. When the collection period is set to 3 hours, the maximum water depths in the storage tank during the rainfall events on July 27, 2024, and September 11, 2024, are reduced by 51.8% and 67.8%, respectively, while maintaining an average pollutant collection efficiency of over 80%. The results provide a valuable reference for the operational management of initial rainwater storage tanks in scenarios where online water quality monitoring is unavailable.