Dextran is a bacterial polysaccharide which can be produced from sucrose and usually used for clinical treatment as one kind of practical polysaceharide drags. The molecular weight distribution of dextran is an important index for its application. However, it is hardly to control strictly by the traditional separating method with ethanol. Taking separation and purification of medicinal dextran as the object of this study, a new separation technology, coupling electro-ultra-filtration with solvadng-out-crystagization (EUF-SOC) was developed and applied successfully, It was found that the permeate flux of optimized EUF-SOC technology is several times of normal ultra-filtration (UF) or enhanced ultrafiltration with an electric field (EUF), and its quality is higher than that of traditional process. The selective crystallization of dextran can be realized and homogeneous dextran crystals can be got in the process of membrane filtration by using EUF-SOC technology. The results showed that the mass transfer coefficient (k) of EUF-SOC process is about 4 times of UF and 2 times of EUF.