Disclosed is a process for the preparation of 11-(4-[2-(2-hydroxyethoxy)ethyl]-1-piperazinyl)-dibenzo[b,f][1,4]thiazepine. In the process, low-priced 2,2′-dithiosalicylic acid as starting material is subjected to bond formation reaction with 1-chloro-2-nitrobenzene in a basic aqueous solution, a nitro group reduction reaction is conducted, cyclization and chlorination reactions are simultaneously carried out in the presence of a equivalent amount of halogenating agent, a reaction with piperazine is continuously conducted without separation, and a reaction with 2-haloethoxyethanol is conducted, thereby it is possible to economically producing Quetiapine, that is, 11-(4-[2-(2-hydroxyethoxy)ethyl]-1-piperazinyl)-dibenzo[b,f][1,4]thiazepine, in an environmentally friendly manner. Particularly, the process is advantageous in that economic efficiency is assured because of use of the low-priced starting material, use of an organic solvent is minimized because a reaction is conducted in an aqueous solution, and it is possible to achieve the environmentally friendly and economical process having high commercial usefulness because the number of reaction steps of the process is reduced and because generation of acidic waste is minimized.