The ion selective film of Nafion was used to modify the iridium oxide electrode, and it is found that the coated ion selective film on the electrode improves the resistance to the interference of oxidative/reductive negative ions. SEM was used to observe the pore size distribution on the surface and cross-section of the coated selective film on the electrode for the purpose of optimizing the preparation process of the film. Cyclic voltammetry experiment was carried out to study the electrochemical behavior of the Nafion-coated electrode. The ion selective mechanism is suggested based on the calculation of positive/negative ion transport coefficient, i. e. t+ and t-, using the method of film potential measurement.