The invention discloses a method for extracting polysaccharide of spirulina platensis. The method comprises the steps that spirulina platensis is subjected to an extraction process to form an extracting solution, the extracting solution is filtered through a microfiltration membrane and is subjected to ultrafilteration, concentration and drying, then more than one type of crude polysaccharide of aqueous extract spirulina platensis is obtained, the content of total sugar is larger than or equal to 10%, and the molecular weight is more than 5,000; or the extracting solution is concentrated and decontaminated, a processed solution is centrifuged under the rotating speed lower than or equal to 15,000 r/min, filtered through the microfiltration membrane and subjected to ultrafilteration by a filter material with the molecular weight in a range from 5,000 to 200,000, when no effluent flows out from an outlet of an ultrafilter, the effluent of the ultrafilter and a concentration solution in the ultrafilter are collected to be dried, then more than one type of polysaccharide of aqueous extract spirulina platensis is fromed, the content of total sugar is larger than or equal to 40%, and the molecular weight is in a range from 5,000 to 200,000. According to the method for extracting polysaccharide of spirulina platensis, reasonable processing measures are selected at different stages of a proces route according to different requirements for product purity, and then high purity olysaccharide with the scheduled molecular weight specification can be formed without detection; and moreover, polysaccharide of spirulina platensis with a pluraity of specifications can be formed simultaneosuly. Accordingly, the method has the advanatags which are impossible to be obtained by other traditional methods.