Water hyacinth was modified by citric acid to prepare a carboxyl-functionalized water hyacinth (CWH) adsorption material. The effects of pH, contact time, amount of the adsorbent, initial concentration, and temperature on the adsorption of heavy metal ions were investigated. In the experimental study, the optimal pH values of Ni2+, Cu2+, and Cr6+ in the CWH adsorption solution were 7.5, 6.0, and 5.0; the optimum contact times were 300, 180, and 120 min; and the optimum adsorbent dosages were 0.65, 0.50, and 0.40 g, respectively. The adsorption processes of Ni2+, Cu2+, and Cr6+ by CWH are in accordance with the quasi-second-order kinetic model and Langmuir model, with the maximum adsorption capacity being 59.64, 77.98, and 96.89 mg/g, respectively. The thermodynamic model shows that it is an endothermic spontaneous process. Even after five adsorption-desorption cycles, the removal ratios of Ni2+, Cu2+, and Cr6+ by CWH remained about 55.23, 65.63, and 71.46%, respectively. The structure of the water hyacinth before and after modification was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy. The results indicate that the surface of CWH was rough and the number of microporous increased and water hyacinth has been successfully carboxylated. The abovementioned results indicate that CWH can be used as an environmentally friendly and economical adsorbent.