The invention discloses a method for extracting water soluble beta glucan from fruit bodies of hericium erinaceus, which comprises the steps of: adopting the fruit bodies of the hericium erinaceus as an extraction raw material; performing alkali extraction, ethanol deposition, alpha amylase hydrolysis and acid hydrolysis; and drying a precipitate after the ethanol deposition to obtain the water soluble beta glucan. The method also solves the problem of poor water solubility of beta glucan when alpha glucan is removed; polysaccharide obtained by the method is soluble beta glucan, the polysaccharide, namely the beta glucan of the fruit bodies of the hericium erinaceus has high content, and the average molecular weights are between 2,000 and 200, 000 daltons; and the water soluble beta glucan has excellent water solubility and can be used as a raw material for preparing medicaments, health care products and food.