The selective fluorination of aramid fiber surface was reported by Fe3+ coordination on the benzimidazole unit. After the coordination, both of the two parts in benzimidazole unit, benzene ring and imidazole ring, take priority in the reaction with the fluorine gas, while the reaction with the amide bond is restrained. In this way, selective fluorination on benzimidazole unit is achieved, with the surface polarity increasing and chain scission behavior suppressed simultaneously. What's more, the increased surface polarity enables the improvement of the interfacial shear strength by 24%, compared with that in a traditional fluorination way. In all, the study offers an easy and effective strategy for the regulation and controlling of direct fluorination reaction, and it provides a guideline for further adjustment of the reaction mechanism to meet the requirement in other applications.