Sudden water pollution accidents threaten the safety of people's drinking water, such as accidental or deliberate contamination events. In this contribution, we introduce a real-time early warning system that can monitor water to ensure safety. We report results of a pilot-scale installation of a sensor-based early warning system (EWS) to detect and report water quality problems along the Yangtze River near Nanjing, China. The system used four different sensors. Water quality parameters detected in this study were necessary to calculate water quality indices consistent with the Chinese government standards. Sensors transmitted data to a server, which stored them in a database, integrated the data into different water quality indices, and sent them to the client. An alarm was triggered if the indices exceeded a certain threshold. The originality of this article was that we set the indices' thresholds and the combined threshold of them to judge the sudden water pollution accidents clearly and accurately and then we gave the alarm levels according to thresholds. The client can support the EWS by storing the data, facilitating inquiries about historic data, and presenting oscillograms and trends within the database.