NOVELTY - Preparing biogas by anaerobic fermentation of agricultural straw involves selecting and processing raw materials, and carrying out anaerobic fermentation to produce biogas. The raw material is agricultural straw, inoculum, nitrogen, biomass carbon (biochar) and water. Agricultural straw is selected mixing wheat straw, corn stover, rice straw, sorghum straw, cotton rod, pole beans, peanut vine, corn silage, bagasse or straw Jerusalem artichoke or any two or more of them by weight. The activated sludge inoculum produces methane and biogas residues.    USE - Method for preparing biogas by anaerobic fermentation of agricultural straw (claimed).    ADVANTAGE - The method enables to prepare biogas by utilizing waste, agricultural residues and sewage sludge in a simple, cost-effective and eco-friendly manner.    DETAILED DESCRIPTION - Preparing biogas by anaerobic fermentation of agricultural straw involves selecting and processing raw materials, and carrying out anaerobic fermentation to produce biogas. The raw material is agricultural straw, inoculum, nitrogen, biomass carbon (biochar) and water. Agricultural straw is selected mixing wheat straw, corn stover, rice straw, sorghum straw, cotton rod, pole beans, peanut vine, corn silage, bagasse or straw Jerusalem artichoke or any two or more of them by weight. The activated sludge inoculum produces methane and biogas residues. Nitrogen is selected by mixing inorganic nitrogen from urea, ammonium, nitrate or ammonia or organic nitrogen beef extract, peptone, yeast extract, corn steep liquor, fish meal, peanut meal or soybean meal. The agricultural straw and inoculum are present in ratio 1:0.5-2 by weight. The ratio of mixture of agricultural straw and inoculum with water 10-30:70-90 by weight. Biochar and nitrogen are present in ratio 20-35:1. Mixture comprises 1-10 wt.% biochar and 90-99 wt.% agricultural straw. The raw materials are processed by mixing agricultural straw pulverized to 2cm, inoculum pulverized to 2cm diameter uniformly with water. The biomass reactor is charged with biochar and nitrogen. The crushed agricultural straw, inoculum and water are added, mixed and sealed for anaerobic fermentation at 25-65 degrees C for more than 35 days to obtain high yield of biogas.