The invention provides a novel preparation method of 2-mercaptoisobutyric acid. Raw materials used in the method comprise 2-ethyl bromoisobutyrate, thiourea, an alkali solution, a solvent, and a hydrolyzate. Compared with other synthetic methods, the method provided by the invention has the advantages of a high yield, high purity, cheap and easy-availability reagents, mild reaction conditions, a simple technological process and environmental friendliness.