As a main reservoir of Haihe River Basin in Henan Province, the water quality of Zhangwu Reservoir is poor in recent years due to natural and human factors. To strengthen the reservoirs' water environmental safety and to ensure the water quality, the Zhangwu Reservoir was taken as research object, the indexes system of its' water environmental safety were stablished using the DPSIR model, the weight of each index was determined by the entropy value method and the rank of water environmental safety in 2015 was evaluated with cloud model. The results showed that the rank of water environment of the reservoir was relative safe, but there were potential safety hazards. There were many abandoned industrial and mining enterprises around the reservoir, and the industrial waste water and agricultural chemical fertilizer and pesticides were discharged into the reservoir with rainfall and irrigation withdrawal water, coupled with the traditional cage farming mode, which made the concentration of TN and TP severely exceed the standard, and the water quality of the reservoir was V kind throughout the year, which the reservoir management department needs to pay more attention. We conclude that the cloud model can reduce the uncertainty during evaluation process and its evaluation results were consistent with the reality, which can be used to evaluate water environmental safety.