The electron‐deficient ester group substitution in the sidechain of the commonly used electron‐withdrawing quinoxaline (Qx) unit is seldom studied, while ester‐substituted Qx units possess easy syntheses and facile modulation of the polymer solubility, and the enhanced electron‐withdrawing property of ester substituted Qx unit can theoretically broaden the optical absorption of the resulting polymers and improve the open circuit voltage in the corresponding organic solar cells (OSCs). In this work, a novel ester‐substituted Qx‐based narrow bandgap polymer (NBG) donor material PBDTT‐EFQx, which exhibits an absorption edge of 790 nm (bandgap < 1.6 eV), is designed and synthesized. Results show that the OSCs composed of PBDTT‐EFQx and PC71BM present the highest power conversion efficiency (PCE) of 6.8%, compared to PCEs of 5.0% for PBDTT‐EFQx:ITIC based devices and 4.1% for PBDTT‐EFQx:N2200 based devices, respectively. Characterizations and analyses indicate that the PC71BM‐based OSCs have well‐matched energy levels, better complementary light absorption, the highest and most balanced carrier mobilities, as well as the lowest degree of recombination losses, and therefore, leading to the highest PCE among the three types of OSCs. This work reveals that the ester‐substituted quinoxaline unit is one of the potential building blocks for NBG polymer donors. Ester‐substituted quinoxaline (Qx) unit affords a novel narrow bandgap polymer donor material PBDTT‐EFQx, which shows broad light absorption to 790 nm. PBDTT‐EFQx gives complementary optical absorption, well‐matched energy levels, high and balanced carrier mobilities, as well as low degree of recombination losses, leading to a high power conversion efficiency of 6.8% in the PC71BM‐based devices.