The utilization efficiency of single atoms is limited by the active sites being buried in support. To maximize atom utilization, Cu single atoms are anchored on the external surface of g-C3N4 nanofibers (CNNFs), which are fabricated by partial thermal decomposition and subsequent self-assembly in H2O. The abundant N and O-containing functional groups of CNNFs are vital for the loading of Cu. Benefited from the full utilization of Cu sites, CNNF-Cu exhibits excellent Fenton-like catalytic activity for Methylene Blue degradation. The reaction kinetic constant is more than 10 times higher than that of the traditional single-atom catalyst with Cu sites evenly distributed throughout the entire support. CNNF-Cu maintains most of its catalytic activity after five cycle tests, suggesting good stability. This work provides a convenient method to maximize the utilization of single metal atoms for catalytic applications.