The invention discloses an acid degradation method of Hericium erinaceus polysaccharide. The method comprises the followings steps: dissolving: Hericium erinaceus polysaccharide is dissolved in water,in order to prepare a solution; hydrolyzation: acid is added into an aqueous solution, and hydrolysis is carried out; neutralization: after hydrolysis, the solution is immediately cooled, an alkali solution is neutralized, and a pH value is adjusted to neutral value; centrifugation: the solution obtained in the step (3) is centrifuged; dialysis: centrifuged supernatant is dialyzed; freeze drying:acid hydrolysis products of the Hericium erinaceus polysaccharide are obtained. The Hericium erinaceus polysaccharide fragments have good dissolvability and high activity; and the Hericium erinaceuspolysaccharide can stimulate macrophage, improve releasing amount of NO, enhance phagocytic ability of macrophage, perform immunocompetence, and have important application values for enhancing immunity of foodstuff and functional foodstuff, and preparing medicament.