The invention discloses an alpha lipoic acid biosynthesis method, an engineered strain and a preparation method thereof. The biosynthesis method is as below: cloning an Escherichia coli lipD gene, an lplA gene, a metK gene and a lipA lipoic acid synthase gene; constructing a lipoic acid structural domain protein overexpression vector; cloning lplA into the vector highly expressing structural domain protein in series way structure to ensure full caprylyl; constructing a lipA expression vector or expression vector in series connection with metK; transforming the 4 vectors into appropriate Escherichia coli strains, regulating and comparing the medium composition and culture conditions, and promoting the conversion of caprylyl structural domain protein to lipoyl. The preparation method of alpha-lipoic acid provided by the invention employs combined low value chemical raw materials and pure biological production method, and has the advantages of little toxic substances in the production process, less pollution to the environment, high safety of the synthesized lipoic acid and high yield; and the alpha-lipoic acid has high activity.