Abstract: Magnetic biochar (FBC) is an adsorption material with good adsorption performance and magnetic separation. However, there are few studies on whether the amount of magnetic precursor in the preparation process and the aging effect affect its structure and the ability to adsorb heavy metals. FBC with different iron-carbon ratios (1∶4, 1∶2 and 1∶1, denoted as FBC-4, FBC-2 and FBC-1) were prepared from rice straw and iron salts, and their surface morphology, functional groups and other physical and chemical properties, as well as the adsorption properties of Cd²⁺ were compared. Two physical aging methods (spontaneous aging and high temperature aging) were used to study the effect of aging on the physicochemical properties and adsorption properties of magnetic biochar. The results showed that compared with ordinary biochar (BC), FBC had a larger specific surface area and pore volume, the number of oxygen-containing functional groups increased, and the characteristic peaks of Fe—O appeared. The saturation magnetization of FBC-4, FBC-2 and FBC-1 increased with the increase of iron content per unit biochar, which were 0.64, 2.21 and 17.69 A·m²/kg, respectively. The adsorption isotherms and kinetic curves of BC and FBC for Cd²⁺ were consistent with the Langmuir equation and the pseudo-second-order kinetic equation. The relationship between the fitted equilibrium adsorption capacity and the theoretical maximum adsorption capacity was FBC-1>FBC-4>FBC-2>BC, that is, magnetic modification could improve the equilibrium adsorption capacity of Cd²⁺, and FBC-1 had a stronger adsorption capacity for Cd²⁺. After two months of spontaneous aging and high temperature aging, the specific surface area of FBC-1 decreased by 45.8% and 36.4%, the average pore size increased by 72.7% and 43.2%, and the saturation magnetization increased to 33.53 and 26.65 A·m²/kg, respectively. Aging could inhibit the adsorption capacity of magnetic biochar for Cd²⁺. The equilibrium adsorption capacity decreased from 36.97 mg/g before aging to 30.97 mg/g (spontaneous aging) and 28.22 mg/g (high temperature aging), and the theoretical maximum adsorption capacity decreased from 63.80 mg/g to 46.68 mg/g and 40.29 mg/g, respectively. Compared with spontaneous aging, high temperature aging had a more obvious inhibitory effect on the adsorption performance of magnetic biochar for Cd²⁺.