Abstract: A dynamic differential equation of membrane diffusion, based on the steady state model, was proposed. The thermodynamic and dynamic specific properties of the electrode have been considered, and an interpretation was given out. The calculated values from the equation agree well with the experimental results; therefore it can be used to solve the problem of response rate and to determine the dynamic parameter of the electrode, The theory can also be used to explain the behaviour of the other gas sensing electrode with gas-permeable membrane.