This research is to understand the effect of curtain grouting for earth-rock dam and the regularity of seepage pressure variation after reinforcement. We analyzed the monitoring data of a clay-core earth-rock dam by means of stepwise regression and finite elements method. The stepwise regression analysis indicated that the main factors which affect the seepage field are reservoir water level, rainfall and ageing. The influence of reservoir water level on seepage decreased toward downstream gradually while the influence of rainfall and ageing increased. According to computation through finite elements method, partial curtain can reduce seepage by 10.7%, whereas fully closed curtain can reduce seepage by 37.6%. Full curtain grouting performs better in seepage control.