Magnetic composite microspheres containing carboxyl groups could be activated by thionylchloride. The carboxy microspheres produced reactive acid chloride groups which then react with the free amino groups of enzyme to give peptide bonds (-CO-NH-). Papain was mostly immobilized on the surface of the activated magnetic carriers by covalent binding. Freundlich isothermal binding model was established, the factors involved with the relative activity and the enzymic properties of the immobilized papain were studied in comparison with its soluble counterpart, for which casein was chosen as a substrate. The results showed that the pH, thermal and storage stabilities of the immobilized papain were higher than those of the soluble one. Apart from these, the Michaelis–Menten kinetic constants were evaluated for the immobilized papain and the soluble enzyme. In a word the immobilized papain exhibited better environmental adaptability and reusability than the soluble one.