The invention discloses a method for producing acid through middle-temperature co-fermentation of corn stalks with sludge after short-time and efficient NaOH pretreatment, and belongs to the technical field of the solid waste treatment. The method can produce the volatile short-chain fatty acid by performing the middle-temperature co-fermentation after performing the NaOH pretreatment on the surplus sludge. The method specifically comprises the following steps: first performing pretreatment on the corn stalks, taking the pre-treated corn stalks as the external carbon source, and performing NaOH pretreatment to obtain the pre-treated sludge and corn stalks; feeding the pretreated sludge and corn stalks into a middle-temperature anaerobic static digestion device to perform the co-fermentation, thereby producing the volatile short-chain fatty acid. By performing the short-time and efficient NaOH pretreatment on the surplus sludge, the NaOH pretreatment is in favor of damaging the sludge floc and the cell wall, and promoting the effective release of the organic matter in the cell; the pretreated sludge is co-fermented with the corn stalks, so that the sludge biodegradability and the anaerobic fermentation acid-producing efficiency are further improved, and the reduction and the recycling of the surplus sludge and the corn stalks are realized at the same time.