Abstract: For promoting the work of reservoir water environment safety and strengthening the water quality protection of water source areas, Suyahu Reservoir was as an example, the indexes system of reservoir water environment safety evaluation were stablished using DPSIR model, the weight of each index was determined by the entropy value method and the rank of water environmental safety was evaluated by cloud model in the reservoir in 2015, the results show that the rank of water environment was relative security for the reservoir, but there were pitfalls. The concentration and storage capacity of TN and TP were increasing due to sewage of residential life, the waste water of industry and catering industry from upstream flowing into the reservoir with rainfall and runoff, as well as the impact of human activities such as sightseeing tourism, to make the water quality of the reservoir difficultly match with the standard. Therefore, attention should be paid to the reservoir management department. Conclusion: cloud model can reduce the uncertainty in the evaluation process and realize the reasonable transformation between the concept of water environment safety and its quantitative value, which makes the evaluation result accord with reality.