Nanofiltration tests of mixed aqueous solutions of sodium chloride and sulfate sodium were carried out by employing two kinds of home-made membrane elements with different properties. By using the error minimization algorithm, the specific values of two kinds of salt separation in the nanofiltration membrane produced water under the condition of charge conservation were obtained. The orthogonal design test was used to analyze the different effects of salt content and temperature of feed water on the effect of salt separation. And the main factors of negative desalination sodium chloride in nanofiltration membrane process were identified.