Abstract: The impermeability of PVC-P geomembranes is of significant importance to the safe operation of a project. To avoid the drawbacks of adopting the permeability coefficient to characterize permeability traditionally, a mathematical model for porosity and seepage discharge is proposed based on the results of the vertical permeability tests and the porosity obtained from the low-field NMR tests, and the applicability of porosity to evaluate the permeability is explored. The results show that the low-field NMR technology with 1H atoms as the probe can accurately measure the distribution of pores and pore radiis. The proportions of the micropores, mesopores and macropores and the shrinkage and development of the pore radii are primarily responsible for the variation of the porosity. The porosity is closely correlated with the seepage discharge, and the proposed model can accurately predict the seepage discharge. Furthermore, the porosity can evaluate the impermeability of PVC-P geomembranes.