The invention discloses a method for improving vitamin D content in a cordyceps sinensis calcium strengthening agent by ultraviolet irradiation, which comprises the following steps of: a, preparing a culture medium: preparing a liquid culture medium from a carbon source, a nitrogen source, mineral elements and the like, and sterilizing the prepared culture medium; b, inoculating cordyceps sinensis fungus into the culture medium, and performing shake culture to prepare liquid cordyceps sinensis fungus; c, inoculating the cordyceps sinensis fungus into the culture medium, performing the shake culture under certain conditions, and taking fermentation liquor out; and d, selecting a UV-B ultraviolet light source with the power of 14 to 60 W and the wavelength of 280 to 325 nm, pouring the cordyceps sinensis fungus fermentation liquor into a large culture dish, placing the ultraviolet light source over the culture dish, irradiating the prepared fermentation liquor, concentrating and drying the fermentation liquor, and breaking wall and grinding the obtained product into a powdery product. The method has the advantages of simple process, convenient operation and wide application to prevention and treatment of rickets of children, acalcerosis of women and osteoporosis of old peoples.