Geomembrane leakage caused by construction and other reasons is very common, but the calculation of leakage is a major challenge in engineering construction and operation. Through laboratory experiment, this study tested the leakage of the defected geomembrane at different thickness of high density polyethylene (HDPE) film (w=0.5,1.0mm), different defect aperture (φ=3,5,10mm), and different pressure head (h=0.3,0.5,0.65,0.8m), and then established a mathematical model. The fitted results show that: The defect aperture and pressure head had a greater influence on the leakage than the film thickness. The leakage could be expressed as the power function of the defect aperture and pressure head, but the coefficients in different working conditions were different; therefore, it should be determined experimentally for the actual project.