NOVELTY - Silage method of crop straw, involves (a) crushing freshly harvested green straws to the length of 10-30 mm, (b) mixing the crushed straws with the silage additive, where the silage additive is nitric acid, and acetic acid or ammonia water solution, the mass concentration of the additive solution is 2-10%, and the weight ratio of the additive solution to the straw is 1:100-1:1500, and (c) preserving closed mixture for 30-90 days under anaerobic condition.    USE - The silage method is useful for crop straw which is used for preparing biogas (all claimed).    ADVANTAGE - The method enables to reduce utilization of saccharides by microbes in environment in ensiling process, restrain autologous lignifications process of the straws, and retain content of free sugars in green straws at maximum degree. The most organic matters in the ensiled straws are transferred to liquid phase through two steps of treatment, then the liquid is anaerobically fermented to produce the biogas, and efficiency of anaerobic fermentation and the utilization ratio of the straws are greatly improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for preparing biogas using the above-mentioned silage straw, which involves (a) mixing the silage straws with water in a weight ratio of 1:5-1:20, immersing the mixture at room temperature for 2-8 hours, (b) performing solid-liquid separation for the immersed straw, (c) performing hydrothermal reaction of obtained solid with catalyst, where the catalyst is sulfuric acid, and nitric acid, hydrochloric acid or ferric salt solution, the weight concentration of the acid solution is 0.5-2%, the mole concentration of the ferric salt is 0.01-0.05 mol/l, the weight ratio of the catalyst solution to the straw is 1:200-1:25, the reaction temperature is 80-200 degrees C and reaction time is 5-180 minutes, (d) performing solid liquid separation of obtained mixture, and (e) delivering the obtained liquid of step (d) and obtained liquid of step (b) to anaerobic fermenting tank to produce biogas.