In aramid fiber-reinforced composites manufacturing, para-aramid fiber requires surface modification to improve its interfacial adhesion with matrix materials. In this study, aramid fiber was modified with dilute sulfuric acid, which was gradually concentrated under microwave irradiation. Results showed that the aramid fiber could be efficiently modified. Sulfonic acid groups were introduced on the surface of the aramid fiber, as confirmed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The breaking strength and thermal stability of the modified aramid fiber were not adversely affected. When the concentration of sulfuric acid was 30.0 g/L, the breaking strength of the aramid fiber remained at 90.7% of the original value, and the number of sulfonic acid groups of 1 g modified aramid fibers was 1.38 x 10^–5 mol/g. Thermogravimetric and X-ray diffraction analyses showed that the main structure of the aramid fibers was not affected. The aramid fiber surface was not severely etched, as evidenced by scanning electron microscopy images. Therefore, this modifying method involving a gradual change in sulfuric acid concentration may be widely applied in many fields.