Recently, much attention has been focused on the hydrothermal carbonization processes that use biomass as a carbon source to produce functional carbonaceous materials. Despite much progress in their synthesis and applications, flexible and high mechanical strength hydrothermal carbonaceous materials are in demand in practical environmental applications. In this work, a flexible hydrothermal carbon-coated electrospun PAN fiber membrane was developed. The carbon coated PAN fibers showed an enhanced mechanical strength and were found to possess a good adsorption capacity toward paraquat. After the NaOH activation, the adsorption process became more efficient. The hydrothermal carbon coating and activation reactions were confirmed by SEM observations, TEM images, FT-IR spectra, Raman spectra, XPS analysis, and mechanical tests. The adsorption process followed the Langmuir isotherm and pseudo-second-order kinetic model; meanwhile, the activated adsorbent (AC-PAN) had a good recydability and the removal efficiency remained at 83% after five cycles. The maximum adsorption capacity for AC-PAN was 437.64 mg/g, which was higher than those in classic paraquat adsorbents. Given the high adsorption and regenerability performance of the activated hydrothermal carbon-coated electrospun PAN fibers, they should have potential applications in water remediation.