The invention relates to a method for preparing organic-inorganic mixed fertilizers from corn stalks by using pulping black liquid by a KOH and NaOH mixed soda pulping process, and belongs to the field of paper making and fertilizer production. The method comprises the following process work procedures that corn stalk raw materials are prepared; corn stalk raw materials are fed into a roller type straw cutting machine to be cut; after the cut, stalk fragments enter a dust remover to be subjected to dust removal; the materials are conveyed by a scraper conveyor; the materials are fed into a spiral pre-soaking machine to be soaked; intermittent steaming and boiling is performed in a steaming ball; the materials are fed into a spraying and blowing cabin to be subjected to thick paste spraying and blowing; black liquid is extruded through a single-spiral slurry extruding machine; the black liquid is stored by an underground slurry storage pool; the black liquid is conveyed into an organic fertilizer workshop for preparing the organic fertilizer. The organic-inorganic mixed fertilizer prepared by the invention belongs to the organic fertilizer with various fertilizer effects; the fertilizer effects are high; the functions are complete.