The invention relates to the field of lentinus edodes, and provides a method for rapidly extracting polysaccharide from lentinus edodes. The method is characterized in that lentinus edodes or stems thereof are subjected to crushing, soaking, flash extracting, extract liquid centrifuging, deproteinizing, micro-filtrating, reverse osmosis, and microwave vacuum drying, such that lentinus edodes polysaccharide is obtained. The method comprises the specific steps that: raw material selection and pretreatment are carried out, wherein dried lentinus edodes or stems thereof are crushed and soaked; flash extracting is carried out, wherein the lentinus edodes soaking liquid is extracted by using a flash extractor; centrifugation is carried out, wherein the extract liquid is centrifuged, such that a supernatant is obtained; deproteinizing is carried out, wherein neutral protease is added into the supernatant, and enzyme-method deproteinizing is carried out; micro-filtrating is carried out, wherein suspended solids such as crude fiber and pectin are removed from the deproteinized liquid by using a 0.2-0.5mum microfiltration membrane; reverse osmosis concentration is carried out, wherein the micro-filtrated liquid is subjected to reverse osmosis concentration; and microwave vacuum drying is carried out, wherein the polysaccharide concentrated liquid is subjected to microwave vacuum drying, such that the lentinus edodes polysaccharide is obtained. The technical scheme provided by the invention has reasonable process configuration, such that operation time is effectively shortened, and operation efficiency is improved. Process linkage automation degree is high, such that semi-continuous operation can be realized. No organic solvent pollution is caused, and the method is environment-friendly. The method has low integral operation energy consumption, and is economical and feasible.