To address urban challenges such as waterlogging, water pollution, and water scarcity, this study proposes a design method for rapid rainwater filtration systems based on rainfall return periods. The objective is to purify stormwater and reduce pollutant loads through filtration. The physical design of the rapid filter must account not only for water quality treatment targets, filter media selection, and operational strategies, but also for factors such as rainfall intensity and catchment area. Rainfall intensity is determined by the return period. Using the Chicago Hydrograph Method, rainfall intensity is calculated for different return periods and design storm durations. This intensity, combined with the runoff coefficient and catchment area, is used to estimate the peak runoff rate, from which the minimum required cross-sectional area of the filter is derived. A case study conducted in Nanjing demonstrated that, across various return periods, the runoff flow from a given catchment area consistently peaked at 47 minutes after the onset of rainfall. This peak flow condition represents the maximum treatment demand on the filter, and thus serves as the basis for determining its physical dimensions. Under specified conditions—a 10-year return period, 120-minute storm duration, and a catchment area of 200 m²—the corresponding cross-sectional areas of the rapid filter were calculated for different filtration velocities (v = 1, 2, 3, and 4 mm/s). The results were 10. 344 m², 5. 172 m², 3. 448 m², and 2. 586 m², respectively. These findings provide a methodological reference for the design parameters of rapid rainwater filters, and contribute to the advancement of rooftop stormwater utilization technologies and the improvement of urban stormwater management systems in China.