Exploring cost-effective and efficient metal-free electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the development of energy conversion and storage technologies. Reported here is a novel heterocyclization strategy to construct efficient ORR catalysts based on linear conjugated polymers (LCPs), which are composed of N-, S-, or Se-heterocycles. Among these polymers, the covalently linked pyridine and thiophene molecule (P-T) with reduced graphene oxide (rGO) exhibits a remarkable half-wave potential of 0.79 V (vs. RHE) and excellent electrochemical stability, which are among the highest values for metal-free polymers as ORR catalysts. Density-functional theory (DFT) calculations reveal that the molecule with a phenyl unit (P-Ph) is catalytically inactive, and when a thiophene unit is introduced to replace the phenyl unit in the conjugated backbone it features highly efficient electrocatalytic active sites. More importantly, the well-defined molecular structures and controllable active sites in the pyrolysis and metal-free polymers highlight new opportunities for the catalytic metal-free ORR.