Method for synthesizing dextran with controllable molecular weight step by step through single enzyme method and application of dextran

The invention discloses a method for synthesizing dextran with controllable molecular weight step by step by a single enzyme method and application of the dextran, a dextran product is synthesized by adding a precursor to control the molecular weight, and the product types comprise dextran T70, dextran T40, dextran T20 and dextran T10. The method comprises the following three steps of: (1) adding dextransucrase and cane sugar into maltose or isomaltooligosacharide serving as a receptor, or preparing dextran with the weight-average molecular weight smaller than that of the target dextran through membrane separation to serve as a precursor; (2) adding the precursor prepared in the step (1) into a sucrose solution, and then adding dextransucrase for synthesis conversion; and (3) purifying the dextran-containing solution obtained in the step (2), firstly removing impurities, then carrying out two-stage membrane separation and spray drying or carrying out ethanol precipitation separation, and then drying to obtain