In this study, dextran polysaccharide was used as a hydrophilic non-solvent additive at different molecular weights and concentrations to prepare novel flat sheet PES membranes through non-solvent induced phase separation process. Formation mechanisms of the PES membranes were assessed thermodynamically and kinetically to verify the influence of dextran polysaccharide on evolving porous media. The obtained results revealed that, in the presence of dextran, the thermodynamic instability of the dope solutions could be remarkably enhanced, leading to a rapid instantaneous demixing during the phase inversion process. Morphologically, the long channel-like macrovoids of the sublayer were also transformed to a disordered structure and concave shapes were emerged on the membrane surface at high concentrations of dextran polysaccharide. Based on the obtained results, not only significant enhancements were achieved in membrane fluxes, but also BSA protein rejections and antifouling properties were improved by increasing the dextran concentration at either molecular weight. Accordingly, the experimental results indicated that the dextran polysaccharide could be employed in practice to achieve scalable high-quality UF membranes through simple NIPS method.