Search for environment-friendly phosphate adsorption materials in polluted water to realize the high-value utilization of agricultural wastes corn straw under mild hydrothermal conditions, the effects of KOH and FeCl3 modification on the corn straw hydrochar were investigated with corn straw as raw material. And Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), XRD, and XPS were used to characterize the surface functional groups, structure, element content, and morphology of the modified hydrochar. The adsorption mechanism of phosphate in water was explored through adsorption kinetics and adsorption thermodynamics tests. The results showed that the adsorption kinetics of phosphate on the modified hydrochar conformed to the quasi-second order kinetic equation (R2>0.95, P ≤ 0.05), and the adsorption thermodynamics conformed to the Langmuir equation (R2 ≥ 0.94, P ≤ 0.05). The adsorption of phosphate was a spontaneous endothermic reaction (ΔGθ＜0, ΔHθ＞0) and monolayer adsorption and controlled by rapid reaction. Both FeCl3 and KOH modified hydrochar can improve the ability to adsorb phosphate, and the adsorption mechanism was different. The main reason of FeCl3 modified hydrochar can adsorb phosphate was that it has good electrostatic attraction. After KOH modification, phosphate adsorption mainly depended on large specific surface area and ion exchange. The corn straw hydrochar modified with FeCl3 had a large adsorption capacity for phosphate, and the maximum adsorption capacity at 45 ℃ was 2.25 mg/g, which can be used as a potential adsorption material for phosphate in polluted water.