Developing novel electron-deficient building blocks (acceptors) plays a vital role in conjugated polymers for n-type field-effect transistors. Incorporation of lactam moieties on conjugated backbones has been one of the most important design concepts of acceptor units, whereas the potential of lactone-fused structures has seldom been explored in polymer semiconductors. Pechmann dyes are a kind of lactone-based electron-accepting units. Theoretical calculations prove that these lactone-based acceptors possess higher electron affinity than their lactam counterparts. In this work, 6,6-endo-dilactone-based Pechmann dyes were successfully incorporated in conjugated polymers for the first time, providing high electron mobilities of up to 0.51 cm2 V−1 s−1. These results demonstrated the high potential of lactone-fused structures as electron-deficient building blocks for conjugated polymers. Furthermore, different alkyl chain substitution positions were found to significantly affect the energy levels and intermolecular interactions, and consequently the device performance, indicating the important role of side chain engineering in polymer semiconductors.