Abstract: Leakage significantly impacts the operational and maintenance safety of reservoirs. Research on defective geomembrane leakage remains largely experimental, with limited exploration of numerical simulation methods specifically focused on impermeable reservoirs with defective geomembranes. Addressing the research challenges associated with defective geomembrane leakage, a simulation method was proposed based on the seepage characteristics of geomembrane defects. Based on an analysis of 1360 days of reservoir leakage monitoring data, a three-dimensional reservoir geomembrane defect seepage model was developed. The permeability coefficients of the defective geomembrane anti-seepage layer were estimated and validated using data from ten processes of operational reservoirs. The validation shows that the research method is reasonable and reveals that with the operation of the reservoir to the dynamic high reservoir level, the permeability coefficient of the impermeable layer of the defective geomembrane consists of three phases of decreasing, increasing and stabilizing, which is in line with the law of infiltration and deformation of coarse-grained soil under the geomembrane and corresponds to the three phases of the seepage pressure and the seepage pressure drop in the monitoring points under the geomembrane. Research results provide technical support for the evaluation of safe operation of geomembrane impermeable reservoirs.