Polythiophene (PTh) is an important conjugated polymer for its simple structure, high stability, and various optoelectronic properties. However, due to the high oxidation potential, it is challenging for the unsubstituted thiophene to be polymerized and deposited onto insulating substrates in large area. Here, a modified sequential solution polymerization (SSP) method is demonstrated to be applicable for the in-situ preparation of large-area PTh film on either glass or flexible polyethylene terephthalate (PET) substrate. Both FTIR and Raman spectra suggest that the thiophene units in SSP PTh are mainly linked by C-alpha-C-alpha coupling without overoxidation, providing tight intermolecular pi-pi stacking in the solid state. Besides, the PTh film shows blue/red color when in doped/neutral state, which can be manipulated either chemically or electrochemically, thus having great potential in reversible chemochromism, electrochromism, or sensor applications.