The Donnan equilibrium model of single salt solution and mixed salt solution of charged mosaic membrane is derived, and it is pointed out that the charged mosaic film has a low retention rate for inorganic electrolytes, which can realize the effective separation of electrolyte and low-molecular organic matter. For an ideal charged mounting film, the electrolyte energy passes freely; In the mixed salt system, the partition coefficient of ions may be greater than 1, resulting in the negative retention rate of ions. The Donnan equilibrium model can qualitatively predict the truncation relationship of electrolytes, and judge the positive and loaded electrical properties of the membrane from the retention rate relationship.