Aiming at the prominent problems of stability and leakage of geomembrane in plain reservoirs, a seepage finite element model was established in combination with a practical project of a plain reservoir with geomembrane in the Pearl River Delta. The damage and defects of geomembrane during construction and operation were simulated to analyze the influence of geomembrane defects on the seepage stability and leakage at the bottom of the reservoir. The results show that when there is no water in the plain reservoir, the geomembrane defects have little influence on the seepage field and the stability of the geomembrane at the bottom of the plain reservoir. When the water level inside the reservoir or in the outside river is in a high level, the pressure head under the defect membrane is the highest, but the geomembrane can still be stable. When the geomembrane defect is 30 m away from the blind drainage pipe and the diameter of the defect hole is 0.5 to 3.0 cm, the diameter variation of the defect hole has little effect on the leakage. However, the number and location of defects have a significant influence on the total leakage. Keywords: plain reservoir; geomembrane; defects; seepage field 〖FL