The invention relates to a method for extracting lentinan. The method comprises the following steps: (1) performing coarse grinding; (2) performing superfine grinding; (3) extracting according to a high-pressure synergic microwave method: adding superfine mushroom powder into deionized water and extracting for 1-2h according to the synergic microwave method under the conditions that the pressure is 0.12-0.15mpa and the temperature is 110-121 DEG C, thereby obtaining an extracting solution; (4) performing suction filtration and concentrating: performing suction filtration on the extracting solution obtained in the step 3), reducing pressure and concentrating till the volume is 1/3-1/4 of the original volume; and (5) performing alcohol precipitation on the obtained concentrated solution and drying, thereby obtaining lentinan. According to the invention, compared with the traditional hot water extraction method for extracting lentinan, the method for extracting lentinan by adopting superfine grinding and synergic microwave high-pressure extraction has the advantages of short extraction time, high extraction ratio, high effective constituent activity, low production cost, energy-saving and consumption-reducing effects, simple and convenient operation, and the like.