Electro-Fenton is a promising advanced oxidation process for wastewater treatment. In this study, mesoporous carbon grafted activated carbon fibers (ACF@OMC) with different pore size were fabricated by using boric acid as the expanding agent. To study the pore structure and pore size effect on the electro-Fenton (E-Fenton) efficiency of ACF@OMC cathode materials, disordered mesoporous carbon grafted activated carbon fibers (ACF@DMC) were also prepared, and both anionic dye (Orange II) and cationic dye (Rhodamine B) were selected as target organic pollutants. Meanwhile, electro-induced generation rates of hydrogen peroxide and hydroxyl radical were measured. It was found that the E-Fenton efficiency of the cathode materials follows the increasing order of ACF@DMC < [email protected] < [email protected]. Even after 10 consecutive experiments, the reactivity of [email protected] cathode materials almost remained unchanged, indicating its promising application in practical wastewater treatment. This study gives new insights into design and preparation of carbonous materials with superior efficiency.