A three-dimensional finite element numerical model was constructed to simulate a defected geomembrane on a slope based on the two-electrode method. The potential distribution near the surface defect was obtained and the potential distribution in different detection line directions was analyzed. The impact of the detection line direction and position on the leak detection sensitivity was further investigated. The results indicate that the positive electrode should be separated from the defect to prevent its shielding effect on the defect. The distance between the detection line and the defect center should not exceed 0. 2 m if the detection line connected to the main electrode is in the direction perpendicular to the defect. The distance between the detection line and the defect center should not exceed 0. 1m if the detection line connected to the main electrode is in the direction parallel to the defect.