The present invention relates to a method for acquiring a fermented product or a fruiting body of Cordyceps militaris with fermentation of sweet potato residues. The method comprises that a mother strain of Cordyceps militaris is transferred to a culture medium comprising potatoes, glucose, soybean powder, potassium dihydrogen phosphate, magnesium sulfate and agar to culture mature and robust hyphae. A shaking flask liquid is used to culture a second class fungus, and in order to expand seed sources and shorten seed culture periods, a culture medium is the same as that of the mother strain except that no agar is added. The hyphae that grow vigorously can be used as liquid strains. After being washed, filtered, ventilated and dried, sweet potato residues are mixed with rice grains, wheat grains or corn kernels in proportion; then the mixture is added as a Cordyceps militaris fermentation material to the Cordyceps militaris culture medium; after the substance is cooled in an aseptic environment, the culture medium is inoculated with the liquid strains of Cordyceps militaris for light tight culture in a static manner; then the color of hyphae of Cordyceps militaris is changed and mycelium and a fermented product are acquired; the mycelium and the fermented product are dried in low temperature and preserved in bags. The fermented product is used as a raw material of health functional food; and the fermented product is fermented into a fruiting body of Cordyceps militaris, wherein the surface of the fruiting body is densely covered with perithecia, and the fruiting body is dried in the shade and bagged, to be used as a raw material of Cordyceps militaris. In addition, the method provided by the invention illustrates that the plantation effect of corn kernels is better than the plantation effects of rice grains and wheat grain.