In order to provide references for dam seepage treatment，this paper analyzes the leakage reasons of an arch dam downstream surface. Based on the measured data of dam safety monitoring system, the effects of dam deformation, quality defects and low temperature and high water level of unfavorable load on dam leakage were studied. The results showed that, the main cause of dam leakage in low temperature and high water level conditions was horizontal construction joint open and throughout the upstream and downstream. We suggested to put the reservoir level below 1688m in elevation during low temperature period of winter and use the hydrophobic polyurethane (Cyanogen Coagulation) chemical grouting method to process the horizontal construction joint which existed along the perfoliate crack and permeable part.The results of the study can provide a reference for similar project problems.