The invention discloses a method for preparing plant starch glucan by a weak electrolysis method. The problems that an existing glucan production technology is immature, difficult in classification, high in energy consumption and the like are solved. The method for preparing plant starch glucan comprises the steps that plant starch is used to prepare a plant starch solution and the plant starch solution is added to an electrolytic cell for electrolysis, wherein a direct current electrolysis method is used, the voltage is 15-45V, the initial temperature of electrolysis is 25 DEG C, the electrolysis time is 1-6h, and the concentration of an inorganic salt electrolyte is 1-5%(wt); and after the electrolysis, the pH value of the solution is adjusted to 6-7, precipitation is further performed,and the precipitate is collected and is subjected to alcohol washing and drying to obtain the plant starch glucan. The method for preparing plant starch glucan is simple in process, easy in classification, low in consumption, high in output, high in yield, low in raw material cost and low in production cost, and can obtain glucan products with different molecular weight.