In this paper, we studied the assessment method of water environment security for water source reservoir, constructed the indicator system of water environment safety evaluation for drinking water source reservoir based on the driving force-pressurestatus-impact-risk model (DPSIR), and the indicator system of integrated assessment contained 5 program levels and 11 indicators. Then, we applied the model to assess the water environment security of the Yuecheng Reservoir. The results show that, (1) the driving force is the precipitation, the pressure is the pollution load of entering quantity, the status is the water quality parameters, such as TN, TP, and CODMn, the impact is the water qualification rate, and the risk is the N/P and water temperature which decided by eutrophic risk; (2) The Yuecheng Reservoir in April, May, June, and September is in security (the range of integrated index is 0.32-0.38); but due to the impact of pollution load caused by direct surface runoff, the integrated index is increased in July and August, the security level enter into early warming status. The assessment result is in accordance with the actual water environment security status, so the model can be applied in large drinking water source reservoirs on the Haihe River Basin.