Mercapto- functionalized poly (3- 4 ethylenedioxythiophene) and multi- walled carbon nanotubes(PEDOT- CH₂SH/MWCNTs) were prepared by in- situ oxidative polymerization, and the optimum ratio of thecomposite was obtained by changing the content of carbon nanotubes in the composite. The concentrations of heavy metal ions Cd²⁺ and Pb²⁺ were detected separately and simultaneously by using this composite as electroactive material. SEM, XRD, FT-IR and XPS were used to characterize the structure of the composites, and electrochemical differential pulse voltammetry (DPV) was used to conduct the electrochemical tests. The relationship between structure and electrochemical properties was systematically discussed. The results show that the PEDOT-MeSH/15%MWCNTs composite shows the uniform porous morphology, and enhances the sensitivity, selectivity and detectionstability of heavy metal ions Cd²⁺ and Pb²⁺. When the PEDOT-MeSH/15% MWCNTs was used to detect Cd²⁺ and Pb²⁺ separately, the linear ranges are 2~40 μmol/L and 6~40 μmol/L, and the LOD is 2 μmol/L and 6 μmol/L respectively. When Cd²⁺ and Pb²⁺ were detected simultaneously, the linear range is 8~40 μmol/L and 6~40 μmol/L, and the LOD is 6 μmol/L and 2 μmol/L, respectively. The composite shows the good responsiveness and low detection limit. The research shows that PEDOT- MeSH/15% MWCNTs composite electrode has the low mutual interference and high selectivity.