Abstract: A novel evaluation of the separation performance of nanofiltration (NF) membranes for the mixed electrolytes solution was proposed.In the evaluation, the observed transmission of an ion through a NF membrane was used to express the separation performance of the membrane for the ion in the mixed electrolytes solution, which had a relationship with the total concentration of mixed electrolytes, the equivalent fraction and the species of each ion.Firstly, according to the permeation experiments of NF membrane for some binary electrolytes solutions, the competition coefficients of four anions (F-, Cl-, NO-3 and SO2-4) were obtained.Then verification of the model was carried out in the permeation of some mixed electrolytes solutions ［(1)Na+, Cl- and F-; (2)Na+, K+ and Cl-; (3) Na+, F-, Cl- and NO-3; (4)Na+, Cl-, NO-3 and SO2-4; (5)Na+, F-, Cl-, NO-3 and SO2-4］through one kind of commercial NF membrane (ESNA 1).The investigation results showed that both the total concentration and the equivalent fractions of ions were important parameters affecting the separation performance of NF membrane for the mixed electrolytes solution.The agreement between the model evaluation results and the experimental data indicated that the model is suitable for evaluating the separation performance of three NF membranes for the mixed electrolytes solution with univalent cations.