Increases in seepage through dam body and seepage around dam were found during the impoundment of a clay core dam. In order to investigate into the causes, we carried out prototype monitoring on environmental parameters, seepage through and around dam, seepage pressure, as well as seepage amount, and obtained monitoring results. To further analyze the seepage source, we conducted model test on seepage flow amount. Results revealed that all the factors are significantly affected by reservoir water level, which is a main cause of seepage amount change. Rainfall is a secondary cause. Reservoir water level mainly affects the seepage around dam, and then the seepage through dam foundation. The impact of reservoir water level is instantaneous and in the meantime steady.