The invention relates to a method for preparing corn straw coarse feed by microbial beneficial living bacteria. The method comprises the steps of (1) preparing microbial strains: respectively feeding two strains of bacillus subtilis, one strain of lactobacillus acidophilus, one strain of lactobacillus bulgaricus and one strain of rhodopseudomonas palustris onto corresponding culture mediums, culturing, carrying out expanding propagation, and mixing 20g of bacillus subtilis, 20g of lactobacillus acidophilus, 20g of lactobacillus bulgaricus and 20g of rhodopseudomonas palustris for later use; preparing 100g of cellulase; (2) preparing corn straw: flattening the corn straw, and drawing into hair shape with a wire drawing machine for later use; (3) preparing the corn straw coarse feed: putting 1000g of prepared corn straw into a fermentation container, and evenly spraying 5g of cellulase onto the corn straw; diluting microorganism with well water, then spraying onto the corn straw, and fermenting in an airtight way for 7-10 days; taking out the product, drying and packaging to obtain the corn straw coarse feed. The method is simple in technology and short in fermentation period, and is capable of saving a great deal of grain; the prepared biological straw feed is good in palatability, high in lactic acid content and more in beneficial bacteria.