On-surface coupling in ultra-high vacuum is employed as a versatile approach to synthesize pure polythiophene from the 5,5''-dibromo-2,2':5',2''-terthiophene (DBTT) precursor and the corresponding temperature-dependent step-wise reaction mechanism is systematically studied by scanning tunneling microscopy (STM). After thermal deposition of the precursor onto Au(111) surface that is kept at room temperature, a triangle-like pattern and a linear self-assembled pattern are formed with different molecular coverages through Br···Br···S halogen bonds and Br···Br type-I contact bonds, respectively. In the self-assembled nanostructures, the thiophene units adopt trans-conformation. Mild annealing promotes the structural transition of both nanostructures into ordered zigzag organometallic linear chains with all-cis configured thiophene units connected through coordination bonds to the Au adatoms. Such conformational variety is easily recognized by STM, particularly in the case of DBTT-CH3 with the extra –CH3 signals. The covalently coupled products from DBTT precursor are obtained by further annealing the organometallic intermediate at higher temperatures, which lead to the removal of Au atoms and the formation of ordered polymer chains and disordered polythiophene networks. Further characterization suggests that the reaction mechanism is associated with the Ullmann-type coupling to form the ordered chains as well as the Ullmann-type and dehydrogenative C–C coupling to fabricate the cross-linked network polymer. Compared with the on-surface synthesis process of DBTT on Cu(111) surface, it can be confirmed that the Au adatoms are vital to synthesize the polythiophene. These findings provide important insight into the reaction mechanism of on-surface synthesized pure polythiophene and can potentially be applied to synthesize other functional conjugated polymers.