A new synthetic route for rapidly constructing dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]thiadiazoles via a Na2S promoted thiophene-annulation was developed and successfully applied to cost-effective synthesis of D18-Cl polymer that has attracted a great attention as a highly efficient polymer donor in the field of bulk-heterojunction polymer solar cells. A commercially available 4,7-dihalo-5,6-difluorobenzo[c][1,2,5]thiadiazole was converted into a range of the corresponding 4,7-dialkynlated compounds under Sonogashira reaction condition. Subsequently, Na2S promoted thiophene-annulations on them furnished DTBT and its derivatives in excellent yields within 10 min. With DTBT in hands, D18-Cl was concisely synthesized via two distinct routes respectively, which are greatly benefited from less synthetic steps, mild reaction condition, less synthetic complexity, and high overall yields. The developed protocol provides a straightforward and reliable synthetic tool, which is highly conducive in cutting the synthetic cost and complexity for production of DTBT based organic electronic materials and therefore moving forward the potential commercialization of organic solar cells.