The invention relates to a method for simultaneously extracting polysaccharide and protein from pleurotus eryngii processing a by-product, and belongs to the field of deep processing of agricultural products. The pleurotus eryngii processing by-product means the root part which is cut and has no commercial value and is adhered to a medium in the process of processing harvested mature pleurotus eryngii to obtain freshly-sold pleurotus eryngii. The method comprises: immersing a pleurotus eryngii processing by-product ultrafine powder obtained through pretreatment with ethanol with the concentration of 85% at room temperature, centrifuging, adding a phosphoric acid buffer into the precipitate, performing room-temperature salt dissolving to extract protein, centrifuging, performing ultrafiltration on the supernatant by using a 100 kDa ultrafiltration membrane, performing spray drying to prepare protein, and keeping the retention liquid for usage; and after salt dissolving, taking centrifugation precipitate as a raw material, using hot water to extracting polysaccharide, centrifuging, mixing the supernatant with the above ready retention liquid, performing ethanol precipitation on the condensed liquid for 12 h, centrifuging, baking the precipitate to dry at 60 DEG C, adding water for dissolving, performing ultrafiltration by using a 100 kDa ultrafiltration membrane, and performing freeze drying on the retention liquid to obtain crude polysaccharide. By employing the method of combining hot-water extraction, ultrafiltration membrane separation, spray drying and freeze drying, the polysaccharide yield is 10.69% and the protein yield is 11.23%.