The invention relates to a method for preparing micro-molecular dextran of which the average molecular weight is between 1,500 and 8,000 by the degradation of middle, low and small-molecular dextran. According to the method, catalytic degradation is realized by water serving as a medium and carbon dioxide serving as a catalyst. The method comprises the following step of: adding the middle, low and small-molecular dextran into a beaker containing deionized water, so that a mass ratio (a sugar water ratio) of the dextran to the deionized water is (0.2-0.7):1, and stirring and dissolving at the temperature of between 60 and 70 DEG C for 5 to 30 minutes; transferring a dextran aqueous solution to a high-pressure reaction kettle, replacing air in the high-pressure kettle by using CO2, introducing CO2 of which the pressure is between 0.5 and 5 MPa, reacting at the temperature of between 110 and 200 DEG C for 30 to 500 minutes, and performing water cooling; opening the high-pressure kettle, and pouring a solution; and decoloring, filtering and drying to obtain a product. According to the method, the micro-molecular dextran prepared from water serving as the reaction medium and CO2 serving as the catalyst by one-step reaction is clean and environment-friendly; and a production process is simple and easy to operate, and the discharge of waste gas, waste water and waste residues is avoided in the production process, so that real environment-friendly production is realized.