In an attempt to study the seepage characteristics of geomembrane dam with geomembrane defects and the stability of upstream dam slope under rapid drawdown of reservoir level,a finite element model of damaged geomembrane was established to analyze the seepage factors and stability of dam with different damage parts and sizes of the geomembrane. Results revealed that the higher the damaged position of geomembrane and the larger the size, the higher the phreatic line and the greater the leakage; the impact of damage size was obviously smaller than that of damage position. The phreatic line declined abruptly when the water level of the reservoir reduced to the height of the geomembrane. The initial stability safety factor decreased with the increase of the height and size of the defect position. In general, the safety factor decreased first and then stabilized, yet with a sudden rise when water level reached the geomembrane defect. The results provide a reference for the stability evaluation and treatment of geomembrane dam.