Heavy metal pollution is becoming increasingly serious. Heavy metal pollutants are nonbiodegradable and can be bioenriched through the food chain, and thus, they greatly threaten the environment and human health. Hydrogels, as an ideal adsorbent, have been widely used to treat heavy metal industrial wastewater. Sodium lignosulfonate hydrogel (LS) was prepared by free-radical grafting copolymerization, and nano-FeO particles were loaded in LS by an in-situ precipitation method (FeO@LS). The magnetic properties and adsorption capacity of FeO@LS are closely related to the load capacity of FeO. XRD, FTIR, XPS, SEM, TEM, BET, and TGA analyses of the materials were performed. Subsequently, the removal effect of the typical pollutant Cd in heavy metal-polluted water was studied with FeO@LS as the adsorbent. The influences of the FeO@LS dosage and initial pH were investigated, and the adsorption kinetics and thermodynamics were further explored and discussed. Finally, the adsorption mechanism of FeO@LS on Cd was obtained. Results show that FeO@LS has a more stable spatial network structure than LS, and the pore size, specific surface area and active sites increase. The maximum adsorption capacity can reach 88.00 mg/g when pH = 6 and the dosage of FeO@LS is 1000 mg/L. The adsorption of Cd by FeO@LS conforms to pseudosecond-order kinetics and the Temkin isothermal adsorption model. Further mechanistic investigations show that the sorption of Cd on FeO@LS is mainly attributed to surface complexation, electrostatic attraction and coprecipitation. The coexistence of cations in water will inhibit the adsorption of FeO@LS. FeO@LS has superparamagnetism and a good response to an external magnetic field. The adsorption rate can still reach >60 % after four elutions with NaCl as the eluent. This material can be reused and has good application potential.