The invention discloses a method for improving methane yield of corn straw anaerobic digestion by combined pretreatment of alkali and salt, and belongs to the technical field of anaerobic fermentation of agricultural organic solid wastes. The method comprises the following steps: crushing the corn straw, and carrying out pretreatment the corn straw by employing potassium hydroxide and metal salts as pretreatment agents, wherein the metal salts comprise ferrous chloride, cobalt chloride and nickel chloride; adding water, so that the total mass of the water and the water contained in the corn straw is six times of dry weight of the corn straw; fully stirring, and sealing by a sealing film; carrying out sealed storage at a room temperature for 3 days, and then carrying out anaerobic fermentation, wherein activated sludge is needed in the anaerobic fermentation process; adding activated sludge which is 12g TS relative to 52g of TS corn straw, and adding water to reach 80% of total volume of a reaction device; filling nitrogen, and sealing by virtue of a rubber stopper; and fixing the reactor in a table concentrator, and keeping the constant temperature of the table concentrator at 35+/-2 DEG C. According to the method, the anaerobic fermentation cycle can be shortened; the total gas production volume and the total methane production volume are greatly improved; and the removal rate of substances is significantly improved.