A new type of voltammetric sensor material has been fabricated via a facile self-assembled method. A modified glassy carbon electrode (GCE) by phenylsulfonic groups grafted multi-walled carbon nanotubes (CNT-SO3H) with dye molecules via Langmuir-Blodgett (LB) assembling (CNT-SO3H/dye-LB/GCE) were prepared for detecting trace levels of cadmium (Cd2+) ions by square wave anodic stripping voltammetry. The synergy effect between CNT-SO3H and dye as well as orderly aggregates in composite LB films contributed to greatly enhancing the determination performance. Under selected conditions, voltammetric response of the fabricated electrochemical sensor in 0.1 M acetate buffer solution containing Bi3+ ions for Cd2+ ions was linear with its concentration in the range 0.1 to 1.2 μM, with a detection limit of 0.08 μM. In addition, the preparation process of self-assembled composite film modified electrodes was simple, non-toxic, exhibiting higher sensitivity and potential application prospects in aspects of heavy metal ions detection and environmental analysis.