A method for preparing glycine by using threonine relates to a fermentation process in which threonine is decomposed into glycine and acetaldehyde by aldolase. Glycine and acetyl coenzyme A can be produced in a fermentation process, in which acetaldehyde is reduced into acetyl coenzyme A or an acetyl coenzyme A derivative by acetylating acetaldehyde dehydrogenase; or threonine is dehydrogenated by threonine dehydrogenase to obtain 2-amino-3-ketobutyric acid, which is then ligated by 2-amino-3-ketobutyrate CoAligase to obtain acetyl coenzyme A. Coenzyme A can be converted into an acetyl coenzyme A derivative under different fermentation conditions.