A series of donor-acceptor (D-A) type conjugated polymers based on novel thiophene-fused isoindigo (TII) were designed and synthesized via palladium catalyzed Stille copolymerization. We found that the redesign of the synthesis of brominated TII was necessary, and the alpha-bromination had to be completed at the very beginning, or beta-brominated TII was obtained, which only led to cross conjugated polymers. Once the bromination was introduced at the correct position, fully conjugated D-A type polymers could be obtained. A series of fully conjugated polymers were obtained by Stille coupling polymerization of a-brominated TII with different donors, and among them, copolymers with thiophene (T) and (E)-1,2-bis (3-octylthiophen-2-yl) ethene (TVT-8) showed acceptable solubility and were suitable to fabricate solution-processable organic field-effect transistors (OFETs). Top-gate/bottom-contact (TG/BC) devices were constructed for the polymers to test their OFET performances. Both fully conjugated polymers exhibit two-orders of magnitude higher charge carrier mobilities than the cross conjugated ones, and PTII-T shows balanced electron and hole mobilities and PTII-TVT-8 is a p-type dominated semiconductor. These observations indicated that the developed TII unit that has improved coplanarity can be a promising building block for the construction of highly efficient conjugated polymers for OFET applications.