The built-in electric field of the polymer semiconductors could be regulated by the dipole moment of its building blocks, thereby promoting the separation of photogenerated carriers and achieving efficient solar-driven water splitting. Herein, three perylene diimide (PDI) polymers, namely oPDI, mPDI and pPDI, are synthesized with different phenylenediamine linkers. Notably, the energy level structure, light-harvesting efficiency, and photogenerated carrier separation and migration of polymers are regulated by the orientation of PDI unit. Among them, oPDI enables a large dipole moment and robust built-in electric field, resulting in enhanced solar-driven water splitting performance. Under simulated sunlight irradiation, oPDI exhibits the highest photocurrent of 115.1 mu A cm-2 for photoelectrochemical oxygen evolution, which is 11.5 times that of mPDI, 26.8 times that of pPDI and 104.6 times that of its counterparts PDI monomer at the same conditions. This work provides a strategy for designing polymers by regulating the orientation of structural units to construct efficient solar energy conversion systems. Three perylene diimide (PDI) polymers were designed and synthesized such that the molecular orientation of the PDI units was regulated to create and modulate their built-in electric fields. Due to the large dipole moment and interfacial electric field, oPDI enables an extraordinary photocurrent density of 115.1 mu A & sdot; cm-2, which is 11.5 and 26.8 times that of mPDI and pPDI, respectively.image