Abstract: The ultra-micro disc-electrode is combined by adhering with 12-carbon fibers of 7 μm in diameter ,after that this electrode is modified by polyaniline in electrolytic polymerization. The absorption characteristics and the electrode process of cadmium ion on this electrode are studied by cyclic voltammetry, step-scan voltammetry, two-step chronocoulometry and AC impedance method. There are two reduction peaks in cyclic voltammetric curve at －0.90 V and －1.06 V, respectively(Fig.1).Experiments and theory show that the electro-active adsorptive surface complex of Cd2＋ is formed from chemical adsorption of Cd2＋ by polyaniline at the electrode surface. So ,this kind of surface complex rather than Cd2＋is reduced firstly .The potential of the reduction peak is at －0.90 V, which is more positive than the directly reduced potential (－0.98 V) of Cd2＋.There is no corresponding oxide peak appearing in cyclic scanning. The reaction mechanism of electrode process is inferred according to experimental data, which proves that the characteristics of reduction peak are simultaneously controlled by diffusing and surface complex reactions. The theoretic curve calculated by this mechanism is agreement with the experimental curve determined by step-scan voltammetry (Fig.2).The formation constant (K=924) of surface adsorptive complex, the maximum adsorptive capacity(Γo=2.08×10－9 mol•cm－2), the kinetic parameters（kf =4.04×10－3 mol－1•cm3•s－1, kb =4.37×10－6 s－1） of surface complex reaction, the diffusion coefficient（D=8.7×10－6 cm2•s－1）of Cd2＋ ,the thickness of diffusion layer （δd=2.75×10－4 cm）and reaction layer（δk=5.14×10－5 cm） are calculated. Experiment also proves that the reduction peak at －1.06 V is a catalytic reduction peak of hydrogen ion, formed by inductive effect of reduced surface complex of Cd2＋.