NOVELTY - The method comprises performing hydrolytic acidification for common crop straw and then refluxing acid fermentation liquor to perform acidification pre-treatment for the crop straw, performing hydrolytic acidification fermentation for the crop straw after pre-treatment, putting the crop straw after pretreatment into a hydrolytic acidification pond for the hydrolytic acidification fermentation, greatly quickening the hydrolytic acidification speed and efficiency of the crop straw after pre-treatment, and generating a great number of volatile fatty acid after hydrolytic acidification.    USE - The method is useful for performing reflux treatment for crop straw by acid fermentation liquor.    ADVANTAGE - The method improves the gas production rate and the two-phase anaerobic digestion efficiency of the straw in an economical manner.    DETAILED DESCRIPTION - The method comprises performing hydrolytic acidification for common crop straw and then refluxing acid fermentation liquor to perform acidification pre-treatment for the crop straw, performing hydrolytic acidification fermentation for the crop straw after pre-treatment, putting the crop straw after pretreatment into a hydrolytic acidification pond for the hydrolytic acidification fermentation, greatly quickening the hydrolytic acidification speed and efficiency of the crop straw after pre-treatment, generating a great number of volatile fatty acid after hydrolytic acidification, and cooling the pH value of the fermentation liquor of 2-4, where a mass percentage concentration of the straw is 20-40 wt.% and the hydraulic retention time is 30-60 hours. The crop straw is prepared by crushing the straw by a mechanical crusher and then sieving, storing the sieved straw on a ventilating and drying position for storage for standby, where a size of the straw powder is 20-50 meshes, weighing the sieved straw powder, and soaking the straw in the hydrolytic acid fermentation liquor of the straw according to the solid-liquid mass ratio of (1-10):(1-20), where the acidification pre-treatment time is 20-40 hours. A total chemical oxygen requirement in the crop straw liquid increases 3000-1000 mg/l and the substances such as celluloses in the liquid phase increase remarkably. An INDEPENDENT CLAIM is included for a method of producing methane.