For present conventional rain garden systems， the runoff control is realized mainly by building rainwater retention facilities such as green roofs， which leads to poor runoff control due to lack of analysis approach to drainage pipe topology. In this regard， an online monitoring-based runoff control method for rain garden systems is proposed. The method uses sensing devices to realize real-time monitoring of runoff volume and adds water storage functions. Based on the monitoring results， the differences in drainage pipe topology are quantified and analyzed， and the runoff volume control is realized by adjusting parameters such as simulated precipitation and storage volume. In the experiments， the proposed method is validated for its regulation performance. The results show that the proposed method has a high attenuation rate of surface runoff when it is used to control the runoff volume， and has a better runoff volume control capability.