A one-dimensional model is used to investigate the reaction-diffusion process of air plasma with a plasma source by numerical simulation in low pressure open atmospheric environment. The spatial evolutions of main charged species’ densities in the chemical reaction, drift and diffusion processes are obtained for a given plasma flow. The results of numerical simulation are combined with an approximate analytical formula to estimate the necessary plasma flow for electron density when a stable state is formed, with which the necessary power of the plasma source also can be estimated.