The carbon cloth fiber (CCF) has superior conductivity, which can be served as an alternative free-standing carrier in electrocatalysis such as water oxidation. However, the inert surface of CCF making it has weak affinity to its supported catalysts. Therefore, it is necessary to improve the affinity of carbon cloth fiber to improve the catalytic efficiency and durability. Herein, we firstly activated the carbon cloth fiber with a moderate electrochemical activation strategy. The activated carbon cloth fiber (ACCF) has abundant oxygenic groups (–COOH, –OH, =O etc.), which has superior adsorption capacity of transition-metal-based catalysts like Ni²⁺ ions. Subsequently, the saturated adsorbed Ni²⁺ ion on ACCF can be in-situ conversed ultrasmall or atomically dispersed Ni with the assistance of low-temperature calcination under a reduction atmosphere (Ni-ACCF). Ultimately, the Ni-ACCF enables high-efficiency water oxidation activity. The η₁₀ that Ni-ACCF delivered is as low as 238 mV, which is much lower than that of Ni-CCF (324 mV). Furthermore, the Ni-ACCF delivers a η₁₀ of 245 mV at the end of a constant current electrolysis. The electrochemical activation strategy also has superior universality, which can be extended to incorporate other bimetallic or polymetallic transition-metal into ACCF. This work also provides a valuable reference for the other fiber-like materials such as nanocellulosic materials in electrolysis.