The invention discloses a cordyceps militaris culture medium fermented feed and a preparation method thereof. The preparation method comprises the following steps: adding corn bran, brewer's grain and candy pomace into a fresh cordyceps militaris culture medium, crushing, uniformly mixing, inoculating microorganism lactic acid bacteria and bacillus cereus during the mixing process, regulating the moisture content to be 35-45%, carrying out anaerobic fermentation, so as to obtain the cordyceps militaris culture medium fermented feed after fermentation maturity; the fermented feed can completely replace corn in concentrated feeding stuff for flocks and herds. According to the cordyceps militaris culture medium fermented feed and the preparation method thereof, provided by the invention, the fresh cordyceps militaris culture medium is subjected to anaerobic fermentation treatment, and the fermentation temperature is lower than 40 DEG C, so that high-temperature drying is avoided, function materials in the culture medium are protected, the drying process is avoided, the drying cost is avoided, besides, the fresh cordyceps militaris culture medium can be preserved for a long time under a low pH and anaerobic environment, the production performance of cattle and sheep is improved by the cordyceps militaris culture medium fermented feed after fermentation under the action of the function materials contained in the fermented feed such as cordycepic acid, probiotics, enzyme and an acidifier.