The invention discloses a method of a culture medium formula for increasing content of lentinan. The method comprises the following specific steps: preparing a culture medium, preparing a mother strain, preparing fermentation liquor, adding Fe<2+> with a proper concentration, inoculating, standing and culturing at 25 DEG C for 60 d, and collecting mycelia to extract polysaccharide; and preparing a cultivation bag, adding Fe<2+> with a proper concentration, inoculating, mycelium growing, fruiting, collecting sporocarps, and extracting sporocarp polysaccharide. Exogenous metal ions with a proper concentration are added within optimal induction time, growth of mycelia is promoted, expression quantity and enzyme activity of a key enzyme gene for polysaccharide synthesis are improved, the polysaccharide content of the shiitake mushroom mycelia is obviously improved, meanwhile, Fe <2+> with a proper concentration improves activity of a key enzyme for sporocarp polysaccharide synthesis, and the sporocarp polysaccharide content is increased. The method provides support for optimizing a shiitake mushroom fermentation mode, improving the content of shiitake mushroom mycelia and sporocarp polysaccharide and boosting development of shiitake mushroom processing industry.