The present invention relates to a method for preparing a feed composition, which improves the immunity and bowel function of chickens, by using water containing a mixture of nitrogen oxide water and hydrogen water, and, more specifically, to a method comprising: (S10) a step of preparing each of nitrogen oxide water and hydrogen water so as to prepare water containing a mixture of nitrogen oxide water and hydrogen water; (S20) a first fermentation step of obtaining a dermal fermentation product by inoculating a dermal extraction product, extracted by means of the water containing a mixture of nitrogen oxide water and hydrogen water, with microbial complex bacteria including Saccharomyces cerevisiae and Bacillus subtilis cultured in a medium containing the water containing a mixture of nitrogen oxide water and hydrogen water, and fermenting same; and (S30) a second fermentation step of performing fermentation by mixing a nucleic acid decomposition product and rice bran powder with the dermal fermentation product, wherein, after the first fermentation step, a fermentation control unit controls that the second fermentation step proceeds only if the microbial complex bacteria reaches a predetermined bacterial concentration.