In this study, carboxyl potassium salt polysulfone PSF-COOK60% was selected as an additive and carboxylated polysulfone PSF-COOH60% was identified as a membrane matrix material to prepare mixed matrix membranes via a traditional phase separation method. The viscosity of the casting solutions, as well as the electronegativity, hydrophilicity, and morphologies of the mixed matrix membranes, greatly changed with the addition of PSF-COOK60%. This was because -COOK was more easily ionized than -COOH into -COO-, which affected the charges, conformations, and hydrophilicity of the polymer chains. In particular, the membrane with a PSFCOOK60%:PSF-COOH60% ratio of 3:7 (M3) exhibited a minimum aperture of 1.825 nm and a maximum electronegativity of - 29.9 mV. M3 offered outstanding separation efficiency of dye/salt mixtures as seen in the rejection rates of Congo red (as high as 95.5%) and organic salts (as low as 5.6%), as well as the pure water flux of 153.8 L/m(2).h.bar. Furthermore, M3 had excellent anti-dye-fouling performance and showed wide application conditions. Therefore, M3 is an ideal candidate for separation membranes for the fractionation of dye/salt mixtures. The PSF-COOK and other polymer carboxylates can be used as a new kind of additive to change the morphology and chemical properties of the membranes.