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.