Abstract: This paper ifrstly proposes a network security situation awareness model to forecast the future trend in network security, while the integrity and the conifdentiality of the data information collected by the sensors are guaranteed. By using the method of data fusion combining with the data mining, the system implements the water quality monitoring data fusion and decision based on D-S evidence structure. The paper then proposes a decision method of D-S evidence of monitoring water quality, which can analyze the changes of water quality environment, overcome the uncertainty and instability while processing the data information collected by each sensor. Experiments show that the method is effective.