Taking a geomembrane faced rockfill dam on thick overburden of a mediumsized reservoir as an example，the finite element method is adopted to carry out the threedimensional numerical simulation of seepage field of the dam and foundation，focusing on some influence factors，e.g.，different types，sizes and distributions of geomembrane defects. According to the calculation results of dam head distribution，seepage discharge and seepage gradient，the dam seepage safety is analyzed and evaluated. The results show that when the geomembrane has defects，it mainly affects the partial seepage field of the dam body and locally raises the phreatic line，but has little effect on the foundation and abutments. The seepage discharge through the dam body has also been greatly increased. The cushion layer behind the membrane near the defect plays an antiseepage role，and the corresponding local seepage gradient increases significantly. In general，the seepage prevention design can meet the requirements of normal operation of the dam.