Corn straw, a typical agricultural waste, was directly converted into hydrochar with a yield of 77.56% by hydrothermal carbonization at 140–230 °C for 2 h with a solid–liquid ratio of 1:20. The morphology and surface properties were characterized by elemental analysis, specific surface area and pore size analysis and Fourier transform infrared spectroscopy. The results showed that with the increase of hydrothermal reaction temperature, some physical and chemical properties such as the increase of hydrocarbon content, crystallinity, and specific surface area of hydrochar changed significantly. A series of chemical reactions such as dehydration, decarboxylation, and aromatization occurred in the hydrothermal carbonization process so that the prepared hydrochar had rich oxygen-containing functional groups (-HO, C-O-C, C=O) and unique porous structure made the hydrochar prepared at 170 °C had the best removal effect on Cd²⁺ in solution (5.84 mg/g). These specific conditions could remove Cd²⁺ and greatly improve the adsorption performance. The pseudo-second-order kinetic model and Freundlich isotherm model could better describe the adsorption behavior of Cd²⁺. Therefore, corn straw hydrochar as a potential adsorbent for removing Cd²⁺ from water.