Corn straw was used as raw material, and different concentrations (10% and 30%) of HNO3 were used to modify the samples before/after hydrothermal carbonization. Pb2+ adsorption mechanism of hydrochars modified by HNO3 was studied by the results of the structure characterization, isotherm adsorption model and adsorption kinetics model. The results showed that hydrochars modified by HNO3 could form rich oxygen-containing groups. Straw carbon modified by HNO3 before hydrothermal treatment (10%N-JG and 30%N-JG) presented rough porous surface morphology and developed mesoporous structure, forming three-dimensional disordered large-scale microcrystalline structure. Straw carbon modified by HNO3 after hydrothermal treatment (JG-10%N and JG-30%N) produced a large number of micropores with uniform distribution and similar size, forming three-dimensional ordered smallsize microcrystalline structure. 10%N-JG and 30%N-JG had the best Pb2+ adsorption efficiency in all samples, they could reach adsorption equilibrium at 3.5h and 3h, respectively, and their theoretical maximum adsorption capacities were 247.51mg/g and 280.09mg/g. Pb2+ adsorption of 10%N-JG and 30%N-JG was in accordance with pseudo first and second-order kinetic model and Freundlich isotherm adsorption model, indicating that physical diffusion and chemical adsorption were equally important in Pb2+ adsorption. It was found that hydrochars mainly depended on the chemical adsorption of oxygen-containing functional groups to remove Pb2+ from water. The well-developed mesoporous structure was more conducive to Pb2+ entering into the particles, which increased the capture probability of oxygen groups on the internal channels to Pb2+, and ensured the efficient adsorption of Pb2+ by hydrochars.