The invention relates to the field of edible and medicinal processing, and in particular relates to an extraction method and application of usnea longissima polysaccharide. The extraction method comprises the following steps: by using usnea longissima as a raw material, performing grinding, performing degreasing, performing drying to obtain powder, mixing the powder and water to form a dispersionliquid, adding cellulase into the dispersion liquid, performing enzymolysis and performing heating extraction, separating filter residues, and concentrating the supernatant to obtain a concentrated solution; mixing the concentrated solution and an alcohol, performing alcohol precipitation, performing filtering to separate the precipitate, dissolving the precipitate into water, performing centrifugation, performing dialysis desalting, and performing freeze drying on the product obtained after dialysis desalting to obtain crude sugar; and performing deproteinization treatment on the crude sugarby using a Sevage method to obtain a deproteinized solution, performing adsorption on polysaccharide in the deproteinized solution by using an ion exchange column, performing elution on the polysaccharide, after the elution is completed, performing concentration, performing dialysis desalting, and performing freeze drying to obtain the usnea longissima polysaccharide. The method can extract the high-purity usnea longissima polysaccharide; and the obtained product can be directly used for activating intestinal immunity.