Abstract: Polyacrylonitrile-based high-modulus carbon fibers (HMCFs) have excellent properties such as high specific strength, high specific modulus, and low coefficient of thermal expansion. So far, HMCFs have been widely used in aerospace and high-end sports equipment, etc. However, it is really difficult for HMCFs to effectively bond with the matrix owing to their extremely high surface inertness, which directly affects the performance of the corresponding composites. At present, electrochemical oxidation is one feasible surface modification process of carbon fiber which has been equipped online, whereas there is a lack of systematic research on the surface electrochemical oxidation of HMCFs, especially the oxidation mechanism of HMCF surface oxidized by different electrolyte solutions. In this paper, the electrochemical oxidation of HMCFs in four ammonium solutions with different acid-base characteristics was conducted. Changes in fiber surface structure, mechanical properties and composite interfacial properties before and after treatment were characterized and analyzed in detail. The results showed that after the treatment with ammonium electrolyte solutions, the fiber surface was oxidized together with the introduction of N elements and the generation of nitrogen-containing functional groups. Meantime, the degree of fiber surface disordering and the oxygen content also increased with the enhancement of the acidity and alkalinity of the electrolyte solutions. The fiber modulus increased to varying degrees after electrochemical treatment, while the fiber samples oxidized in NH₄HCO₃ and NH₄H₂PO₄ electrolyte solutions showed an increase in fiber tensile strength after treatment, from 4.21 GPa to 4.82 GPa and 4.75 GPa, respectively, and the interfacial shear strength of their compo-sites also increased by 49.86% and 49.02%, respectively, compared with composites reinforced by untreated fibers. It is demonstrated that moderate oxidative etching during electrochemical oxidation can improve the tensile strength of fibers while modifying the fiber surface.