Method for extracting cysteine or salt thereof from reaction liquid and application of cysteine or salt thereof

The invention discloses a method for extracting cysteine or salt thereof from a reaction liquid, which comprises the following steps: (1) pretreating the reaction liquid containing cysteine to obtain a pretreatment liquid; (2) carrying out cyclization reaction on cysteine in the pretreatment liquid and a compound containing an aldehyde group or a keto functional group, and after the reaction is finished, carrying out separation treatment to obtain a thiazolidine acid intermediate; and (3) carrying out hydrolysis reaction on the thiazolidine acid intermediate in the step (2) in an acid environment, and after the reaction is finished, carrying out post-treatment to obtain the cysteine or the salt thereof. The extraction method is simple in operation process, greatly reduces discharge of waste water and inorganic waste salt, is small in corrosion to equipment and pipelines, and is green, environment-friendly and efficient in the whole process. The invention also provides a preparation method of cysteine or salt