Maintaining some chlorine residual concentration in the water distribution system is the key step to the safety of water quality. In this text, it puts forward a new chlorine decay model after studying the chlorine spatial and temporal decay. Each decay coefficient to the model adopts the dynamic simulation of the computer to estimate. Also a modeling case is presented, it shows that the new model is more practicality and adaptability. On the basic of the water quality model, it builds the early-warning and prediction system, which will have great realistic meanings for improving the water quality and ensuring the safety of the water supply system.