The electrical potential difference at both sides of a membrane when it is separating two solutions of the same electrolyte but different concentrations is called "membrane potential". It is an important tool to study the electrical character and the construction of charged membranes. The Teorell-Meyer-Sievers (TMS) model permits a simple prediction for a symmetrically charged membrane potential. Recently, the TMS model was improved by Wang et al. for an asymmetrically charged nanofiltration membrane by assuming a transient concentration at the interface. On the basis of the current jobs, a new model is initiated for a nonsymmetrical. charged membrane, which considers the actual structure of the asymmetrical nanofiltration membrane and assumes a contact factor to compensate the potential change caused by different layer materials' characteristics. This contact factor can explain the membrane potential change due to die orientation of the membrane, giving a reasonable explanation for this, i.e., for positive orientation: where high concentration faces the dense layer, the contactor factor weakens the effective concentration of fixed group concentration and thus lowers the membrane potential; for negative orientation, where the high concentration faces the porous sublayer, the contactor factor strengthens the effective concentration of fixed group concentration and thus enhances the membrane potential. The new model, as well as the role of contact factor, was verified by the experimental data with several commercial membranes reported in the literature. Furthermore, after introducing the contactor factor, the interfacial concentrations obtained for various cases seem to be more reasonable and better conform to the situation of electrolyte transport through asymmetrical nanofiltration membranes.