The electro-Fenton (EF) process is one of the advanced oxidation processes (AOPs). Graphite felt is widely used as an cathode material for the EF process, and its performance can be improved by surface modification. Active carbon nanotubes (ACNTs) have more oxygen-containing functional groups and better electrochemical properties compared to Multi-wall carbon nanotubes (MWCNTs). In this study, graphite felt was used as the substrate, and composite cathodes were prepared by surface treatment using MWCNT, graphene, and ACNTs. Rhodamine B (RhB) dye decolorization tests were then conducted to investigate the degradation performance of the EF system with different cathodes. The results showed that based on the micromorphology of ACNT, the tubular form of MWCNT was activated into a GR-like flake structure, it was also found that the strength of the oxygen-containing functional groups of ACNT improved significantly. The activated MWCNT/C cathode exhibited a 60-min decolorization rate of 77.28% compared to the unactivated MWCNT/C cathode, whereas the decolorization rate of the ACNT/C cathode increased to 85.01% after activation, which was close to that of the GR/C cathode at 88.55%. In summary, the ACNT/C cathode exhibited degradation efficiency comparable to that of the GR/C cathode.