Carboxymethyl chitosan (CMC) were prepared from the carboxymethylation reaction of chitosan under mild conditions, and magnetic carboxymethyl chitosan nanoparticles (MNPs-CMC) were developed by binding CMC on the surface of magnetic Fe3O4 nanoparticles (MNPs) via EDC/NHS activation. The characterization results indicated that MNPs were successfully modified by CMC and the modified nanoparticles had a good magnetic property. By investigating the adsorption of BSA, it was found that the maximum BSA immobilization capacity was up to 159.51 mg center dot g(-1). The adsorption characteristics of BSA can fit the pseudo-second-order model and Langmuir equation well. Furthermore, a high desorption efficiency of 81.47% was achieved under an alkaline condition (pH 9.6).