A pH-responsive drug release system based on doxorubicin conjugated poly(styrene-alt-maleic anhydride) grafted multi-walled carbon nanotubes (MWNT/PSMA-DOX) was prepared by the reversible addition-fragmentation chain transfer (RAFT) polymerization and acid-cleavable hydrazone linkages. Firstly, multi-walled carbon nanotubes (MWNTs) were modified with a furfuryl derivative RAFT agent by Diels-Alder reaction. The polymerization of styrene and maleic anhydride was carried out in the presence of functionalized MWNTs to afford MWNT/PSMA. The maleic anhydride ring located along the polymer chains was opened by hydrazine subsequently conjugated with doxorubicin via an acid-sensitive hydrazone bond. The composite materials were characterized by FT-IR, H-1-NMR, TGA and TEM analysis. In vitro experiments showed a burst release of doxorubicin at pH 5.0 whereas slight release at pH 7.4.