Organic field-effect transistors have received increased attention because of their unique properties such as low cost and large-area production. Among the organic field-effect transistors, donor-π-acceptor (D-A) conjugated polymers show outstanding advantages such as material design flexibility and solution processability. Herein, a series of pyromellitic diimide-based donor-acceptor conjugated polymers (PPMDI-nT) with different donor/acceptor ratios were developed. Their photophysical and electrochemical properties were fully investigated. Furthermore, the organic field-effect transistors based on PPMDI-nT were fabricated through a solution process. PPMDI-1T is an n-type semiconductor, whereas the other polymers exhibit typical ambipolar transport behaviors. As the donor/acceptor ratios increased, the electron mobilities stayed almost the same, but the hole mobilities increased dramatically. Thus the charge-transport properties of these polymers could be tuned accordingly.